You can use the spin indexer to set your degrees. Say for instance, you want to drill holes around the circumference of a piece of round stock. You find the center of the stock, start out at zero on the plate with all of the holes, then rotate the part and lock it in with the spring loaded pin. You can also use it in a surface grinder to grind the O.D. of a piece of round stock. Normally the spring loaded locking pin will have a neutral position that leaves the pin in the up position, then you can continuously crank the handle, hence turning your surface grinder into an O.D. grinder. It does take some indicating in though to get things square to your magnetic chuck on your grinder, or square to the table in a mill. It's not the best way to grind the O.D. of something, but it's a hell of a lot cheaper than buying an O.D. grinder.
BTW....if you got it for $69 or less than what's posted, you got a hell of a deal.