The purpose of the material creation by Brahma is clearly described herein. Every human being should beget nice children in the womb of his wife, as a sacrifice for the purpose of worshipping the Supreme Personality of Godhead in devotional service. In the Vishnu Purana (3.8.9) it is stated:
Varnasramacaravata purusena parah puman
Vishnur aradhyate pantha nanyat tat-tosa-karanam
“One can worship the Supreme Personality of Godhead, Vishnu, by proper discharge of the principles of varna and ashrama. There is no alternative to pacifying the Lord by execution of the principles of the varnasrama system.” Vishnu worship is the ultimate aim of human life. Those who take the license of married life for sense enjoyment must also take the responsibility to satisfy the Supreme Personality of Godhead, Vishnu, and the first stepping-stone is the varnasrama-dharma system. Varnasrama-dharma is the systematic institution for advancing in the worship of Vishnu. However, if one directly engages in the process of devotional service to the Supreme Personality of Godhead, it may not be necessary to undergo the disciplinary system of varnasrama-dharma. The other sons of Brahma, the Kumaras, directly engaged in devotional service, and thus they had no need to execute the principles of varnasrama-dharma.
Source: A.C. Bhaktivedanta Swami Prabhupada (2014 edition), “Srimad Bhagavatam”, Third Canto, Chapter 13 – Text 11