My use, They take the place of an insulated vest under your jacket. They also allow you to get by with a shorter length jacket rather than a full length coat. In serious cold weather use I really don't want the seam at my waist that happens even with insulated pants, severe cold means a long coat as well over the bibs.
You will normally find that folks will leave the bibs on and lose the coat not the other way around. I seldom wear mine without pants on underneath. If I know good and well it will be an all day thing they go on over long johns. Very popular around here is a pullover hooded sweatshirt with the bibs over then a Detroit style jacket over that.
And for cripes sake buckle both straps and tie your boot laces. [rant off]