Loading and unloading gas can and other stuff is much easier from the rear, as already stated and I always kept a small ratchet strap to secure those gas cans so they dont slide. I'm a big boy and tall and I"m not loading anything heavier that a bag of charcoal or mulch over the side. 10$ at harbor freight or walmart and you can get a 2 pack of small straps. Very handy for anything you haul.Why not? I sit allot of stuff on the rail putting it in and removing it from the bed. 5 gallon gas can is a good example, I just do not have the strength to lift it over the rail when full.
I rarely go over the rail for anything and I have the strength, the bed sides are just too high on all of the trucks anymore.
I put gas cans at the back and use a bungee cord across the handles to keep them there. Same with propane tanks that need filled.So you put a 5 gallon gas can in your pickup, you either tie it in back at the tail gate or you sit it up next to the cab. Odds are you will not take off fast enough for it to slide to the back but you could easily brake hard enough for it to slide forward. To each his own, I put mine over the rail up next to the cab, typically fill 3 at a time. I also load stuff like plows, engines, mower decks and other things. with my loader tractor over the side. It's a truck not the family hauler, We have a 2013 Caravan for that.