This will allow you to mount a small hdd in the area that the floppy drive goes in a dell optiplex sff. It may fit other computers but I only know for sure that it fits the sff dell optiplex with a slim cd rom (780, 755, etc).
Mounting a small hdd saves a boat load of room in these small computers by replacing the full sized hdd. It also reduces heat and increases airflow which these little furnaces really need if you'll be doing any kind of gaming on them.
It also frees up a lot of room for larger graphics cards. You'll still be stuck with single slot low profile but you'll have more room for the fans and heat sink.