No reason besides unlocking duty requirements, endgame raiding and contributing more to your team wherever there isn't an item level synch. Gearing for next expansion isn't too difficult, there are a range of easy options to grab 510-530 loot.

It has the advantage over WoW that gear is not an endless grind for tier pieces and legendries and artifact powers and what not to try and keep up with your friends and guild. It's more relaxed and you can gear an alt for Savage in reasonable time.

Sadly we don't have Mythic+ dungeons.