Raamaayan gives only one reason for Raam Avataar and that is because of the nature of Raavan's Var. Kumbhkarn was already deprived with a proper Var so killing him was not a problem. And as Raavan was to be killed surely, there was no other way to kill him except by a human being, because he had already asked in his Var that he should not be killed by Devtaa, Raakshas, Daitya or Gandharv. He did not include human beings and monkeys etc because they were their food, so he had no fear from them. That is why Vishnu had to appear in the form of a human being.

But Maanas gives several reasons of Raam Avataar. When Shiv Jee is telling Kathaa of Raam to Paarvatee Jee, He says - "Raam's birth are many and His Karm (actions) are many. In every Kalp He takes Avataar and people tell those stories in various ways."

The first reason he gives is the incident of Sanakaadi Muni. The second reason he gives is because of Var to Kashyap Jee. The third reason he tells is of Jalandhar Raakshas and his wife Vrindaa. The fourth reason he gives is of Naarad's Shaap. The fifth reason he gives is Var given to Manu and Shatroopaa. The sixth reason he gives of Braahman's Shaap to Raajaa Prataapbhaanu. The sixth reason is partly the reason of Raavan's birth also.