Re: Captain America: The Winter Soldier
My assumption is that the "useless straps" are part of a harness system built in to Cap's suit, seeing as he's essentially a paratrooper. Obviously it's not designed for real world use, but there's a relatively believable illusion of function.
As for the belt pouches: You don't see him using the bathroom on screen either. Some things won't be shown on screen because they aren't entertaining, not because they're not necessary. I think it's a fair assumption that, as a US soldier, he would carry pretty much exactly what any other GI would carry in their pouches. Compass, maps, socks, multitool, flashlight, spoon and cup, bandages, food/water/ammo.