It is same as Krishna's purpose. To remove the burden of the world.
ekonaviṁśe viṁśatime vṛṣṇiṣu prāpya janmanī rāma-kṛṣṇāv iti bhuvo
bhagavān aharad bharam
In the nineteenth and twentieth incarnations, the Lord advented
Himself as Lord Balarāma and Lord Kṛṣṇa in the family of Vṛṣṇi [the
Yadu dynasty], and by so doing He removed the burden of ...
Is there any mention on Vishnu Avatars that took birth in the past Manvantaras?
Yes, as per Vishnu Purana - Part 3 - Chapter 1, below are Lord Vishnu's avtars in past Manvantara:
Yajna was born in the Swáyambhuva or First Manwantara
Ajita was born in the Swárochisha or Second Manwantara
Satya was born in the Auttami or Third Manwantara
Hari was born in ...