Yeah, I would say that the biggest reasons are:
1. Performance
2. Stability
3. Modularity
4. Customizablity
5. Durability
...But I think what you're really asking (since you've specifically stated what sort of rig you own) is, "What sort of advantages could I expect of a backplate and wing over my Seaquest Balance?"
These are:
1. Stability. A backplate is wide and flat and spreads tank weight (or bouyancy) across your back, so there is absolutely no tank wobble whatsoever. The rig is very secured to your back and feels like a part of you. This feature also allows you to dive doubles, impossible with a Balance.
2. Permenantly adjusted correctly. Once you've adjusted a backplate and wing (depending on the harness that you've chosen), it's always dead-on with it's adjustments, as there's nothing to come unadjusted... So no more screwing around with a rig that doesn't fit quite right.
3. Modularity. With a backplate and wing, you can choose and swap wings out as you see fit, so you can use small travel wings when traveling and when you don't need much lift (very streamlined) and use larger wings when the situation requires it. Again, because of this, doubles are a possibility, whereas they aren't on a Balance.
4. Customizability. You choose whatever accessories you want to put on your backplate and wing, and it all swaps out and changes out according to your needs. Which brings me to...
5. Durability. The basics behind a backplate and wing are a solid, thick metal plate and very heavily-fabriced wings... Held together with very durable webbing. Backplates and wings are pretty tough to hurt, and are vastly more durable than the mostly plastic parts on a "standard BC." When you do screw something up, parts are easily exchanged and swapped out, too... As mentioned above... Because the system's modular.
6. Performance. Spreading the load of a standard 6 lb backplate over your lungs can do amazing things for your trim and bouyancy, not the least of which is a fairly dramatic reduction in the amount of additional lead needed to correctly weight a diver. This action also streamlines and simplifies a diver, with the results being improvements in many areas of diving.
