Srimad-Bhagavatam: Canto 1 - Chapter 1 - Verse 33


अपरे वसुदेवस्य देवक्यां याचितोऽभ्यगात् ।अजस्त्वमस्य क्षेमाय वधाय च सुरद्विषाम् ॥ ३३ ॥


apare vasudevasyadevakyāṁ yācito ’bhyagātajas tvam asya kṣemāyavadhāya ca sura-dviṣām


Others say that since both Vasudeva and Devakī 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.


It is also said that Vasudeva and Devakī, in their previous birth as Sutapā and Pṛśni, 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-gītā that the Lord appears for the welfare of all people of the world and to vanquish the asuras, or the materialistic atheists.