I'm not sure about the standard Tk Canvas 'group', but in Tk::Zinc( an enhanced canvas ) , groups are very important in doing things like rotations and motion. Maybe this would be a good question for comp.lang.perl.tk where those experts can explain the c code behind it all.

For instance, if you tried to rotate a bunch of "tags", they would all rotate independently; but if they are grouped, you can specify the group center point, where they would rotate around.