Just the fact you've beaten it is the main reward, although the other things also gain increased value.
As I said a mount from a heroic end boss has more perceived value than a mount obtained from something easier.
You might not come for the gear, but the perceived value of gear that drops from a difficult boss is greater than the perceived value of gear from an easy boss. Whether you turn up for it or not.
Also, sometimes it can be about the gear, much less so after many years though. I'm pretty sure the very first people to walk get their hands on full tier one were pretty please to have done so, and looked pretty badass in a world before transmog and raid tiers into 2 figures. This effect wears off pretty quickly though, as once you've replaced a couple of gearsets you don't really care.
Some games rely almost solely on it being about the gear, although WoW isn't one of them. I'm not going to pretend that bosses don't drop loot, and that especially end of expansion boss loot is for any purpose other than trying to rank on farm and to say "look I beat Garrosh heroic". Same as I'm not going to pretend that the titles are for anything other than that. They are there to say "look I did a thing".