There's one on each side. Running without them would bring the the hubs inward about 1/16" on each side. Without slipping the hubs in and seeing where the seals would ride, it's hard to say if it would hurt anything. It would change the interface between the splined/threaded collar and lock nut a little, too. If it's been working for you, I wouldn't take it apart to add them. But maybe next time you replace the bearings, put them back in then. Nothing really stands out to me as a major problem with not having them in there, though.
Only thing I can think of is to act as a grease seal. Basically to help keep the grease in by the bearings so it doesn't push into the seal. Other then that..........