Can anybody tell me what this is for?

[Al8236]

I used to have one somewhat similar to that, It is a base to convert an right angle grinder to a chopsaw.
Work not bad but can be a pain to set up. mine was universal but that one looks like it might be brand specific.
Good luck Al

On second look it probably is an universal mount it uses the handle mounts and jamb bolts to position the angle grinder. You will need to find the right bolts to fit your grinder.