whats the point of leveling your mount?

You need it at a certain level to upgrade it into a silver mane, I think its lvl 15. The only other benefit is that it can go on without food longer at higher lvls.

Odd, I seem to remember that lvl being 5... I could be wrong though.

Really, the other benefit is pretty useless anyways since when your mounts hits maximum tiredness, it'll reset the tiredness by 5 and dismount you. Nothing stops you from using your mount again.