Let me add a possibility to the XY stage with post. As already mentioned, it could be part of a sensitive drill press, or an inspection microscope. It could also be part of a watchmaker's staking press. In all three cases, you are missing most of the machine.
But I suspect you were asking about the thing dead center on the portable bench. The answer is: "Whatever the maker wanted to use it for". It is not a standard part or an OEM machine component. It's something somebody made to do a job. And there is no way for us to guess what that job is. It's scrap.