Hindu Bhagwan Brahma gave birth to the demons – Srimad bhagavatam

📖Srimad bhagavatam 3.20-23
Lord Brahmā then gave birth to the demons from his buttocks, and they were very fond of sex. Because they were too lustful, they approached him for copulation.

Why did Brahma give birth to demons and what was the need for him to give birth to demons from his buttocks ?