The outside leg part is to tell the horse which lead you want. When traveling in the confines of a ring, it's the outside leg. Basically, by sliding that leg back a hair, you're "guarding" his outside hind leg from swinging out when you apply in inside leg at the girth to ask for the canter depart. It tells the horse, I want you to start the canter with your outside hind leg. The reason you release the inside rein at the same time is to free up his inside shoulder. Everything in horses work in diagonals. To get the outside hind to act, you have to free up the inside front leg which ultimately frees the corresponding shoulder. Sorry, I seem to be in a wordy mood today. Hope that makes sense.
As for keeping the outside leg there, if you're on a straight line, you don't need to. One a circle, yes because it keeps the haunches from swinging out so you keep the fully body bend that you want for circling.
You just have to see your distance...you don't have to like it.