devakyam yacito ’bhyagat
ajas tvam asya ksemaya
vadhaya ca sura-dvisam
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
Others say that since both Vasudeva and Devaki prayed for You, You have taken Your birth as their son. Undoubtedly You are unborn, yet You take Your birth for their welfare and to kill those who are envious of the demigods.
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
It is also said that Vasudeva and Devaki, in their previous birth as Sutapa and Prsni, underwent a severe type of penance to get the Lord as their son, and as a result of such austerities the Lord appeared as their son. It is already declared in the Bhagavad-gita that the Lord appears for the welfare of all people of the world and to vanquish the asuras, or the materialistic atheists.