OK, I see Bruces link is working now, and it's an excellent description. It appears to offer some numbers with it.

In our modeling application, the tab can reduce the force needed to drive any of our surfaces. With the big models those forces can go way beyond what our servo's can give. And we often wind up with 2 or 3 servos, each of which costs big bucks, working just one aileron. Not good economics. And beyond the money, it's a royal pain. The servos have to be sync'ed for one. Installed for another. A royal pain.

It's actually amazing that they aren't on every big bird of ours. Guess not many model designers know about them. Or maybe the servo mfg's are paying hush money.