1. After gathering enough Fame on area you can buy items in Fame shop (happy face on map). Those items are unique and cannot be obtained in other way.
2. Lots of achievements you can get from killing 'elite' monsters (summoning them from bags or just killing them, if meet on map).
3. Cause jumping is cool. Even if has no purpose ;p
4. Usually only character, which first hit monster get all xp and loot. Just prevention from kill-stealing.