why do you want your back hand to be your dominant hand? because more power can get generated from the rear hand. it is easier to learn how to punch with your dominant hand and so it is easier to deliver a ko blow (which is ultimately the aim of any striking). the front hand is generally used to set up that knockout punch with less powerful strikes that are quicker and expose the defences of the puncher less.
a clear argument there is that if it is possible to generate more power with the rear hand , why not have your weaker hand at the back to compensate for its inherent lack of power and the strong hand in the front to allow quick placement of those powerful strikes? becus it takes a long time to develop ko power in ur front hand and an even longer time to develop ko power in ur non-dominant rear hand. for this reason, if a boxer is very experienced he is able to pull of switching stances or fighting with a dominant lead hand effectively, but if he is not, he only handicaps himself further.