I'd imagine it could work like this:
Sell side: You have running servers with GPUs too weak for economic mining.  You sell time on your GPUs to get in a little extra cash
Buy side: You need to do some work which a GPU is suited to but it doesn't make financial sense to buy one.

The only problem is that the kind of machines likely to be running 24x7 are servers, which don't tend to have GPUs fitted, but if you do... may as well make some money off them!