As I understood it (there was an improvement for it), the wrist guards prevent the fletching in the arrow to harm the wrist, when they are shot...
From the position you shoot an arrow, I suppose the "feathering" of the arrow is likely to go by the wrist of the arm that is holding the bow, and eventually tear it open.
Again, that was my understanding... Not that I actually ever shot a (real) bow (playmobows don't count, right?
)