Lots of the messengers around here that wear kits use them to hold maps, pens, wallets, locks, phones, $$, smokes, anything that you might have had in a pocket, but wouldn't necessarily want to swing your bag around to get at each time you need it. Lycra has no pockets, jersey pockets are less than handy comparably & most long board riders prefer cycling shorts, jersey & naught else during the summer.

Some on the weekends wear them without a mess bag, ride with the hip pouch on the back, works very well, carries small tools & crap but no sweat patch on the back, less jiggly, waterproof & holds more securely than jersey pockets.

Seatbags don't come inside with you & generally aren't waterproof. Fanny Packs are never waterproof, always poorly made of cheap materials and always sag or compress themselves or their belts or both. So hip pouches are substantially different from those things.