I don't think it could be put more succinctly than PJ and ST said it.  I will add this can be applied in a current as well, such as paddling a river or stream.  If there is appreciable current - against you or with you, you want the trim to favor the upstream end, as the lighter end will always want to be ahead in the flow.  Usually not much of an issue in most of the BW/Q creeks and flowages.

pd