One of the points of DF is to create insanely complicated ways of doing both incredibly simple things and incredibly clever stuff...
A refuse stockpile is a stockpile, which is a place to store stuff for (possible) later retrieval, i.e. one use of a general mechanic. In the case of a refuse stockpile you might not want to retrieve things (although the bone carver might be of a different opinion), but the mechanics don't know about that. Storing things in empty air is of course silly, which is why you can't try to do that. A lot of DF consists of fairly general mechanics that are then used for various purposes.
The dumping mechanism can be used to designate the tiles around a hole (and the air over the hole as well, but that tile serves no actual purpose) as a dumping zone, and everything dumped on the rim will fall down into the hole (Also check Loci's reply below). Using dumping for this purpose have two disadvantages, though:
1. Selection of things to dump is manual, so it requires micro management (there is a DFHack autodump feature, but I think that one is for worn clothing only).
2. Using dumping for garbage disposal locks the function up so you can't (easily) use it for other purposes (get rid of that boulder! It's in the way! [but it's often faster to just use that boulder to make a floor]). Clearing out worn clothing is my main usage of the dumping function, as well as getting rid of boulders from mine cart tracks (to avoid road kills when a dorf goes away to pick up a bounder lying on an active track. Can also be done by forbidding the boulder).
Finally, a Quantum Stockpile is actually fairly simple to construct once you understand how, but I readily admit it's definitely not intuitively obvious.
Edit:
Quote from a German mathematician (I've forgotten who): "Warum soll es einfach sein, weil es so wunderschönes kompliziert werden kann?" -> Why should it be simple, when it can be so wonderfully complicated?