I think I understand why Apple offers wheels as an option. They've priced the Mac Pro so high and out of reach of most consumers and even some professionals. In fact, it seems like Mac Pro is really designed for studio environments: movie production houses, sound editing and design studios.
In those studio environments it makes sense to divide the tasks like video editing and sound design into separate rooms specifically optimized for it. So instead of buying two machines, you can buy one and slide it around from room to room based on a specific task. The I/O seems to be super chainable so the transition between rooms would take a couple minutes, whereas it could save hours of rendering time and precious minutes of waiting around for previews.
Based on the above the wheels make sense to me. They look pretty stylish too 😎.
P.S. Unfortunately I can't afford a Mac Pro, but I have enough savings to buy the wheels. Maybe I'll start there 🙃.