Given the same bow set up used each time...............

The more weight you can pull the faster you can shoot a given arrow and the more energy and momentum it will have. Also you can shoot heavier and heavier arrows at the same speed as you once did the lighter arrows and get more energy and momentum from the arrows.

The more energy you have the easier it is to get an arrow all the way through your intended animal. Deer do not require much energy to blow all the way through them. Once you start shooting at elk and moose you want that extra energy.

Same thing goes for draw length. But for us adults that have stopped growing this is a non factor. For growing kids though this is important.