If you are doing the weight check at the beginning of the dive, with a full tank, then at the end of the dive, with an empty tank, you will be at least roughly five pounds lighter. IF you want to be able to control your ascent all the way to the surface, you don't want to be five pounds light there. (Although you most likely will have lost a little lift, due to gear getting saturated with water, wetsuit compression that hasn't totally rebounded, etc. -- and in a dry suit, it may work the other way, because you were totally squeezed, hanging vertically in the water at the beginning, but you almost certainly can't get the suit that empty at the end of the dive.)
So, if you are neutral with a full breath at the beginning of the dive, you're probably several pounds negative with a normal one, which is where you want to start your dive. It's a rough way of determining correct weighting.