You guys are missing the question here. Not asking why shave on show day but asking why during/for a keep.

Here is my guess before I head to bed,

When hosing him/her down after workouts(between sets or after hard sets) to cool down that's when the shaving helps more with cooling basically helps with KEEPS during hot weather. It can also be a method to seeing how the bulldog results are looking throughout his/her keep or 3/4 keep. The fur will regrow on show day. (Weigh daily of course and No shave for show)

About shaving for the show, if bulldog is shaved for show he/she will stay cooler but IF goes to scratching getting wash the furless will dry quicker than the one with fur thus by making the furless the less cooler one now. Cons on furless will have less armor. If no scratch, Pros he'll be the cooler one. Cons will be the same as having less armor no matter what. But to me shaving for show isn't needed and is not important in this case.

And yeah on show day some do it for cosmetic purposes not aware of the pros/cons, this we'll leave aside. Lol now let's dig deeper into shaving during keep.