There are two conditions in which you can buy a salvage auto: repaired or in need of repair. Obviously, the first is easier -- the work is already done, and a salvage auto dealer might have done some of the inspection and licensing legwork for you. If you buy a repaired salvage auto, you might be able to drive it out of the showroom.
However, buying a wrecked car allows you to monitor the repairs yourself. First and foremost, you can ensure that all repairs are done to your standards. Second, the cost-conscious buyer can choose which repairs to have completed and which quirks they're willing to live with. You don't care if the back power window doesn't work? Just pocket the cost of the repair.
Again, buying a salvage auto is sweat equity -- the more work you're willing to do yourself, the more money you'll save.