In terms of items you get from raiding:

- highste ilvl gear available
- weapon with the highste ilvl for the current patchcycle
- dyeable raid-gear (shares the model with the gear from NM, but dyeable)
- mountdrop for clearing the last tier, aka O12S

In terms of other things someone can get from raiding:

- the satisfaction to have cleared some of the most challenging content in the game
- fun, by doing that with friends
- seeing the full, last fight - at least in SB each of the last fights (4, 8 and 12) have a special phase thats exclusive to Savage
- an aneurism from all the anger if people dont show up in time for the third week
- (pathtic) bragging rights, depending on when you clear it