I think it’s mainly for Mac / iOS development CI systems like for CircleCI or Mac Stadium. The hardware just seems too expensive for it to replace more generic arm services that AWS is offering.
On-site music and video production. You can rack it in a portable rack with your other racks of music equipment and it fits in the back of a van.
Well, if your portable rack happens to be way deeper than most audio racks…
If you're wheeling around five to six figures worth of audio production equipment often enough to want the rackmount Mac Pro, I think the cost of a new rack is almost a rounding error.