excuse someone explain to me avatar in the animator?

I am following a guide where the person put the FPC in the controller and left the avatar set to “None.” If I do that, the character stays in the T-pose and does not move. The only way I can get the character to move into the rifle idle position is to set the avatar to "Rifle Aiming Idle.

Will setting Avatar to rifle aiming idle messing with the rest of the amination? As I want to add more animation to the character