New skills, a sens of progression/achievement. Suddenly you have one extra motivation to play.

I mean what's more interesting, grinding a dungeon so you can change outfit once in a while or go explore new areas and venture on heroic quests with real reasons to do things. All while doing so your character become more skillful and powerful and become capable of powerful skills that allow for new tactical advantages and undertake challenges you did not believe possible.

What m trying to say is, its a game changer, it keeps things fresh and supplies you with another long term goal.

I must confess however that I have never really been fond of itemlevel as a way of progression in a game. You are basically removing all progression and long term goals without one. Sure its easier to know what to do because you only need to focus on one thing, but it sure isn't as engaging and rewarding.