"The Raiders under the leadership of Elder Taos at the Sanctuary aren't exactly friendly. But Warlord Gantas has been giving them a rough time, so if you can bring back enough Skulls from Raider battles, you might be able to convince some to fight for you."
The Raider Training Camp allows you to train Raiders from the Sanctuary to fight on your side.
In general, the Allied Raiders are considerably more powerful than Imperial troops. Offsets are the training costs which can include Skulls, Steel or Nanopods. Most of these units also require and to heal.
In the 4.0 Patch, the Allied Dustwalker, Allied Champion and Allied Skirmisher were moved from the Prestigious Academy to the Raider Training Camp.