That's a mobile repair shop, for tanks, trucks, armored vehicles, artillery pieces and other frontline equipment.
Basically when a vehicle is damaged or breaks down, it calls in such a repair truck - provided of course they are not under fire - and in most cases it should be possible to do the repairs on the spot or at least enable the damaged vehicle to limp further back for more intensive repairs.
Alternatively it can scrap a destroyed vehicle for workable parts to be used in other vehicles. Spare parts are always in high demand!