These large cannons were rolled out onto the arena floor when things were getting out of hand.  They fired powerful sonic blasts that were similar to (but larger than) the handheld blasters wielded by the Geonosian warriors.  They can be seen delivering their sonic ordnance around the arena, blowing up objects and knocking Jedi to and fro.  This, along with a Geonosian gunner figure, would be a nice compliment to the Orray w/cart in a case assortment of “Class I vehicles”, or whatever it would be called these days.  It would work great in rounding out the arena battle.