The thumb wheel is for adjusting the center-center distance of the bridge so that it can be fit to frames/forks with slightly different spacing between posts. Also might be useful for applying some preload in the system to help braking performance.
Well how would you solve the problem of mass manufacturing a U-brake with a booster bridge that will fit posts that are 88, 89, 90, or 91 mm apart? Seems like a pretty ingenious solution to me. The other options being to provide an un-drilled booster that is drilled with the proper spacing at the time of installation (e.g. WTB) or possible providing slots instead of holes (e.g. MRC and many others).
Was just that I have two exage mountain versions with no adjustment. Didn't think about the cam on setup and hasn't seemed to make any difference between the two types.