each participant in a losing siege gets a durable supply
http://www.runesdatabase.com/item/206771/durable-supply some honow (500 max I think) and some badge of the trial (think 5 max, they go into currency list automatically). The badge of trial aka sw badges, are used to purchase siege titles, and can be used at pet vendors to buy items.
If you tie or win, you get a glorious victory package, up to 1300 or 1500 honor points, and 13-15 badge of trials.
Though for many, the most important thing is the entertainment you get from duking things out with other players.