It holds all those little things you keep buying then possibly discarding. Or it letsyou buy bigger things. So, for me, coffee filters for the DVC coffeemakers. I could keep buying 100 packs for too-much at the store at my resort, and never use them all and try to remember to bring them home and back (we don't have a coffeemaker that uses the same filters) or I can buy one big pack and leave it in there. Coffee filters aren't going to make up the cost of the service, but if you have enough things you pack and repack and forget etc, it starts adding up, especially in convenience.
We have little handheld fans in there (with the batteries removed of course) we have all our old MBs. Laundry detergent. Some will-last-forever condiment type things, sealed in double ziptop bags of course! Between one trip and the next I left my shorts, because it was winter here and I wouldn't need them and didn't feel like bringing them back home then back to FL. An Igloo cooler backpack, aloe vera, hydrogen peroxide (again double sealed), other first aid stuff... other stuff, too. We don't maximize what we have in there, but others most definitely do. For us it's more about convenience. Because of DH's work travel we generally have status that allows us a couple free bags each, so for us it's not to cut down on cost, but it's to increase the ease.