[Closed] What does this do?

I'd imagine it's some sort of disc brake mount for a frame that doesn't already have them.

I may be wrong, however.

Yep acts as a disc brake mount for frames without, doesn't fit all frames though. Fitted a few and they are a pain in the arse to fit, but work.

Disc mount for frames with slot dropouts. I had one - it's the best (only) solution if you absolutely must run discs on your old slot dropout frame.

I thought it was something like that, but really can't see how it held a calliper securely in place.