1. Importance of Puranas

1.1 Puranas Originated From the Breath of The Supreme God

Brihadaranyaka Upanishad 2.4.10
As clouds of smoke proceed by themselves out of a lighted fire kindled with damp fuel, thus, verily, O Maitreyî, has been breathed forth from this great Being what we have as Rig-veda, Yagur-veda, Sama-veda, Atharvâṅgirasas, Itihâsa (legends), Purâna (cosmogonies), Vidyâ (knowledge), the Upanishads, Slokas (verses), Sûtras (prose rules), Anuvyâkhyânas (glosses), Vyâkhyânas (commentaries). From him alone all these were breathed forth.”  (source)

1.2 Puranas are Wealth, Treasure and Prosperity

Atharva Veda 11.7.22-24
Gain, acquisition, and success, fulness, complete prosperity. Great gain and wealth, are laid, concealed and treasured, in the Residue. All things that breathe the breath of life, all creatures that have eyes to see, All the celestial Gods whose home is heaven sprang from the Residue. Verses, and Songs, and magic hymns, Purana, sacrificial text. All the celestial Gods whose home is heaven sprang from the Residue.”  (source)

1.3 Puranas are the Fifth Vedas

Chandogya Upanishad 7.1.2
Nârada said: ‘I know the Rig-veda, Sir, the Yagur-veda, the Sâma-veda, as the fourth the Âtharvana, as the fifth the Itihâsa-purâna (the Bhârata)”  (source)

Shatapatha Brahmana 13.4.3.13
it is these he instructs- the Purana is the Veda: this it is; thus saying, let him tell some Purana…”  (source)

Shrimad Bhagavatam 3.12.39
Then he created the fifth Veda — the Purāṇas and the histories — from all his mouths, since he could see all the past, present and future.”  (source)

Vatsayana on Nyaya Sutra 4.1.62
It will not be right to regard the said scriptures as having no authority; for the authoritative character of these is vouched for by authoritative texts; as a matter of fact, the authoritative character of Itihāsas and Purāņas is vouched for by Brahmana-texts, which are entirely autho- ritative; e.g., The Atharvangirasas declared the Itihasas and Puranas: and these Itihāsas and Purāņas constitute the fifth of the Vedas.’ (Chandogya Upanishad 3-4-2). For these reasons it is not right to say that the said Itihasas and Puranas are not authoritative. As regards the Dharmasāstra scriptures, if these had no authority, there would be an end to all business among living beings, which would put the whole world into confusion. Secondly, inasmuch as the ‘seers’ and ‘speakers’ are the same, there is no reason why these scriptures should not be authoritative; as a matter of fact, the ‘seers’ and speakers’ of the Itihasas, Puranas and Dharmasastra scriptures are the same as those of the Mantra and Brahmana texts (of the Veda). Thirdly, in as much as there is a restriction in regard to their subject-matter (the said scriptures must be authoritative)…”  (source)

Taittiriya Aranayaka 2.10.1-8
Brahman, itihaasas, Puranas are equivalent to Vedas…”  (source) [Page 100]

1.4 Vedas are Incomplete Without Puranas

Mahabharata 1.1.264
By the aid of history and the Puranas, the Veda may be expounded.”  (source)

Brahmanda Purana 1.170b-171
O Brāhmaṇas, he who knows the four Vedas with their ancillary subjects and the Upaniṣads must increase his knowledge of Veda through Itihāsas and Purāṇas. The Veda is afraid of a man of incomplete knowledge saying “O this man will strike me.””  (source)

1.5 Hell for Censuring the Puranas

Ramacharitmanas 7.99.2
They who find fault with the Vedas by dint of logic are condemned to each hell for a whole Kalpa (cycle).”  (source)

Ramacharitmanas 2.92
There is no virtue equal to truth, as the Vedas, Shastras, and Puranas declare. I have reached this virtue by an easy road : if I abandon it, my disgrace will be published in earth, heaven’ and hell.”  (source)

Skanda Purana 5.2.67.23-31
Even as the men were speaking thus, O lady of renown, a voice issued from the firmament as uttered by me in compassion: “Do not say anything out of the way. The immutable Shrutis should not be censured. The Puraṇas spoken by Brahmā, the creator of the worlds, cannot be otherwise. The atheists who censure the Puraṇas and Dharmaśāstras fall into the terrible hell and remain there till all the living beings are annihilated.”  (source)

Padma Purana 4.1.8-13
For a man who would create an obstacle in the act of the narration of Viṣṇu’s account, there is no escape from hell for a hundred period of Manu. For those who, having heard the account (as narrated) in the Puraṇas, censure or scoff at it, always have the very afflicting hells on their hands (i.e. waiting for them). The sin earned in the previous existences (of him) who desires to listen to the account of Śrīkṛṣṇa, perishes just at that moment only. I do not know what (best) position the man who would devoutly listen to Śrīkṛṣṇa’s account will have on hearing it.”  (source)

2. Importance of Manusmriti

2.1 Whatever Manu had said (Manusmriti) is Medicine

Krishna Yajur Ved 2.2.10.2-3
he should insert the verses of Manu’s; whatever Manu said is medicine; verily he makes medicine for him.”  (source)

2.2 Veer Savarkar and Gowalkar on Manusmriti

“Manusmriti is that scripture which is most worship-able after Vedas for our Hindu Nation and which from ancient times has become the basis of our culture-customs, thought and practice. This book for centuries has codified the spiritual and divine march of our nation. Even today the rules which are followed by crores of Hindus in their lives and practice are based on Manusmriti. Today Manusmriti is Hindu Law. That is fundamental.”  ~Veer Savarkar  (source)

”(Manu) the first, greatest and the wisest lawgiver of mankind.”  ~Gowalkar  (source)

3. Importance of Smritis

Manusmriti 2.10-11
The Veda should be known as the ‘revealed word,’ and the Dharmaśāstra as the ‘recollections’; in all matters, these two do not deserve to be criticised, as it is out op these that Dharma shone forth. If a twice-born person, relying upon the science of dialectics, should disregard these two sources, he should be cast out by good men,—the detractor of the Veda being an infidel.”  (source)

3.1 Smritis are the Eyes of God

Devi Bhagavatam Purana 11.1.21
Nārāyaṇa said :– Śruti and Smriti are the two eyes of God; the Purāṇam is His Heart. Whatever is stated in Śruti, the Smriti and the Purāṇams is Dharma…”  (source)

3.2 Smritis are Commandments of Shiva

Skanda Purana 7.1.24.85-87
We shall carry out all this. But listen to (consider) one matter. Even while we adore in accordance with the direction, if we make use of Sivanirmalya (remnants of the adoration of Siva) it will result in our downfall as it is censured in the Srutis and Smrtis. And the Srutis and Smrtis constitute the two great commands of Rudra. Which fool will transgress them even if the vital airs are at the throat even if death is imminent?”  (source)

3.3 Hell for Censuring the Smritis

Brahma Purana 106.135
There is a hell named Atratiṣṭha. It contains putrescent urine and faeces. Those who censure Smṛtis and the Vedas fall into it with faces downwards.”  (source)

4. Hell for Rejecting the Vedas

Mahabharata 3.31
A rejector of proofs, a slanderer of the interpretation of the Vedic scriptures, a transgressor urged by lust and covetousness, that fool goes to hell.”  (source)

5. All Contradicting Verses are Considered as Dharma

Manusmriti 2.14
Where there is conflict between two Vedic texts, both are held to be Dharma; both have been rightly pronounced by the wise to be Dharma.”  (source)

Gautama Dharmsutra 1.4
If (authorities) of equal force are conflicting, (either may be followed at) pleasure.”  (source)

6. No Salvation for Rejecting Scriptures

Gita 16.23-24
He who discards scriptural injunctions and acts according to his own whims attains neither perfection, nor happiness, nor the supreme destination. One should therefore understand what is duty and what is not duty by the regulations of the scriptures. Knowing such rules and regulations, one should act so that he may gradually be elevated.”  (source)

7. List- 18 Major Puranas

Shrimad Bhagavatam 12.7.22-25
Sages expert in ancient histories have declared that the Puranas, according to their various characteristics, can be divided into eighteen major Purânas and eighteen secondary Purânas. The eighteen major Puranas are the Brahma, Padma, Visnu, Siva, Linga, Garuda, Nárada, Bhagavata, Agni, Skanda, Bhavisya, Brahma-vaivarta, Märkandeya, Vämana, Varaha, Matsya, Kürma and Brahmanda Puranas.”  (source)