Picture this: you're having a good day. After spending well over $50,000 on a specced out Mac Pro, it's finally arrived. You set it up on your desk, boot it up, and about five minutes later, it comes crashing down because the wheels don't have locks on them.
While you probably didn't put it on your desk if you spent the extra $400 on the wheels, the point is the same. As pointed out in YouTuber Marques Brownlee's review, there aren't any locks on the wheels, so it can roll off anywhere.
This might not seem like a major issue; after all, it's not like you're setting it up on top of a volcano where it can roll right into a lava pit (unless you are). But even if the Mac Pro is sitting under your desk on a wooden floor, you're probably going to have to set up something yourself to make sure that the PC doesn't move.
Obviously, for a PC that can run you over $50,000, where just wheels cost an extra $400, this is something that probably shouldn't be an issue.