gooberking: Well I will have to take your word on it, as I'm no expert on them. I heard a lot of people saying they were basically a steam key reseller at this point, but they very well may be as much of an exception to the phenomenon as one can expect to find. From what I can see of GMG on the other hand they are a key seller which is the practice I don't understand. Sales misalignment aside, just buy from Steam if your going to be in it anyway.
GMG also has their own service, the "Capsule" or whatever it is. I think their selling point at least used to be that some Capsuie games could be sold again after use, ie. a second-hand market for digital games.
However, I think GMG has mostly become a Steam (and maybe Origin?) key seller now, lowering their own Capsule service into irrelevancy. I wonder if anyone still buys Capsule games specifically? I have only ever bought a couple of Steam keys from GMG.
The point of these various third-party Steam key sellers is that they should be able to sell Steam keys cheaper than Valve/Steam can themselves, as they don't have to pay anything for the service (Steam) itself, and apparently don't have to give a cut to Valve either (at least for now). So they are a bit like leeches and freeloaders on Valve's expense, but with Valve's blessing. Valve gets even more presence in the PC gaming market with these other key sellers, and at the same time it lessens the interest of these other stores to improve their own infrastructure (GMG Capsule, GamersGate's own download service, Humble Bundle's own service etc.). The more they sell Steam keys, the more they make their own service and infrastructure irrelevant.
Maybe that is also why GOG has decided not to join the ranks of Steam lackeys by start selling/offering also Steam keys, knowing that would weaken their own service (which IMHO has happened to GMG, GamersGate and even HumbleStore, for this very reason). I don't think it takes much to become a Steam key seller that just gathers breadcrumbs from Valve's table, anyone can do it.