In the last generation (and presumably true for this one), Threadripper Pros retained higher single core boost frequencies. Useful for mixed workstation loads, where you might still end up with some applications that are single/low threaded, and the extra boost is noticeable.

Also, since these will mostly sell through OEMs, I imagine many of these might get put together with the whole validated hardware+software package deal thing.

Workstation motherboards are different things to server boards. Threadripper pro blurs the lines a bit but there's a decent chance it still comes with conveniences like audio and a load of USB ports that the server boards won't bother with. The chassis will look more like (and maybe sound more like) a desktop too.

I'm using a single socket epyc milan as my daily driver and it's great. 64 cores and 512gb of memory is is good times. But that only works for me because it's headless and the thin clients talking to it can do things like make noises and connect to bluetooth keyboards.

For me it was just convenience - I bought a complete workstation (tower) with that CPU in it (and with all its capabilities, like 128 PCIe lanes and 8 memory channels). Does the EPYC server chip use a different socket (?), so I'd have to build a tower workstation from the components myself, a server mobo, server CPU squeezed into a tower?

An appropriate cooling system for a 2x Epyc mobo will cause so much noise I'm not sure we could call it a workstation. Well, you could try to, but nobody else in the room will hear you.

Higher boost clocks. And the cheap EPYCs are single-socket too.

The real issue is that there are no non-Pro threadrippers anymore. Those were even cheaper.

AFAIK the Threadripper is basically a prosumer-oriented Epyc with higher boost clocks and a little bit better pricing.