The 19" wheels are intended to improve handling, with lower sidewall tires; sort of stiffens the whole thing up a bit. So the strut tower brace is there to help handle the increased load being transferred into the struts from the stiffer wheels/tires and also further stiffen the chassis.
So the whole package is about crisper handling and the strut tower brace is part of that.
Whether it actually makes any significant difference is an open debate.
Personally I like the idea of it, even if it is not significant, I figure why not? And it looks cool under the hood.
JBert, in your opinion, I'm getting lowering springs for my '06 GT for better handling, and was thinking to add the tower brace as well, so you are saying I would not waste my money if I do add the tower brace.