So, to expand on High Wing's point.....yes, a STA is a solid piece of (normally) metal that holds the tank in EXACTLY the right spot. They add a little extra weight (or a lot, depending) and add some flexibility. For me to switch my bpw to doubles, I'd have to unthread my cambands, unscrew two sex screws, remove my wing, replace my wing, and bolt on doubles. To do it with a STA, you don't have to unthread your cambands. This reduces work, you don't have to keep adjusting them, AND it reduces wear on your cambands (my velcro is already on its way out).
The downsides are that it increases weight (maybe good?) and costs more. Also, it holds the tank further from your back, which reduces stability, especially with big steels. Plenty of wings have built-in "tank stabilizers" or "STAs". I had a Hollis wing with a little tank stabilizer that did great. I've got a Hog wing now that also has those tank stabilizers. I haven't dived it yet, but I won't dive it with a STA. I also don't need one, as I won't be switching to doubles. My wing allows for cambands through the plate (not available on all wing/plate combinations). Some people like STAs, some don't, some don't have an opinion. I don't really have an opinion, but I don't see the need to buy one, so I won't. If I do, which I won't, I will.
Yes, there are certain brand combos you should avoid if you want to dive without a STA.....but I'm not aware of all of them. I can tell you for sure the Hog Wing doesn't fit a Hollis plate without a little dremel work. A STA would've fixed it. The only concern with mixing brands is that the camband slots in the wing may not align with the camband slots in the plate. Halcyon wings don't have camband slots at all.
Here's the best picture I've found of one so far. The "flat" part goes against the plate. The "open" part is what the tank rides on. You screw your wing/plate/STA combo together with two screws, like you would without the STA. STAs come in all shapes and sizes. Have a Google around the web and you'll see a few designs.