The only thing I can possibly imagine would be some seriously high-end 3D rendering or massively parallel simulation software.
To the OPs point, there is zero reason to get the Mac Pro over even a Mac Mini.
Seems like there's a hole in Apple's lineup. You can't get a discrete GPU without a monitor attached (iMac) or spending >$3000. Not that it matters much for Illustrator, Photoshop, etc. which are not GPU-parallelizable anyway. At least not yet.