Lord Vishnu became the son of Vasudeva in order to reduce the burden of the earth

दुर्वासश्चरित्रवर्णनं नामैकोन्विम्शोऽध्यायः

Durvasa character description Chapter 19

नन्दीश्वर उवाच |

Nandishwara said (to Sanatkumara)

ब्रह्मप्रार्थनया विष्णुर्वसुदेवसुतोऽभवत् |
धराभारावतारार्थं साधूनां रक्षणाय च || 57 ||
हत्वा दुष्टान्महापापान्ब्रह्मद्रोहकरान्खलान् |
ररक्ष निखिलान्साधून्ब्राह्मणान्कृष्णनामभाक् || 58 ||

By the prayer of Brahma, Lord Vishnu became the son of Vasudeva in order to reduce the burden of the earth and to protect the good people.
Having killed evil people, people with large sins, bad people who hate Brahmins, (he) bearing the name Krishna protected all the good people and Brahmins.