In the Gita, Lord Krishna Himself explains His incarnations:
You and I have passed through many births.
I know them all, but you do not remember.
I am born from time to time
whenever the good need my protection.
I am born to destroy the bad and help the good.
My birth is divine and those who understand this become part of
Me and do not have to be born again.
The Gita, Chapter 4, The Sword of Knowledge, Verses 5-9
These beautifully succinct verse encapsulates Hinduism’s fundamental beliefs: the existence of God, His powers to create and destroy, God’s benevolent intent, the importance of understanding divine power, the reality of reincarnation, and the meaning of salvation which is becoming one with God.
See The Gita, a New Translation of Hindu Sacred Scripture, by Irina Gajjar.