It's possible that seeing Bruce do this gives Matilda courage. He becomes a role model when it comes to standing up to adults and fighting for the rights of kids everywhere. It's as if he looked the Trunchbull right in the eye and said, bring it on. Do your worst, you old meanie. This seems to be the moment that Matilda embraces this idea of fighting back with hope of success. After all, if Bruce (a silly, over-eating boy) can stand up to the Trunchbull in such a big way, why can't she?
After the Trunchbull smashes the cake platter over his head, that's about the last we see of Bruce. We can remember him fondly, especially as portrayed in Quentin Blake's illustration, sitting on the stage in the assembly hall, absolutely replete and stuffed to an inch of his life with chocolate cake. (For more about Bruce and what his eating of the cake represents, head over to the section "Symbols, Imagery, Allegory.")