Smaller size for easier storage and transport
Mini PCs are in an entirely different realm compared to SFF and tower PCs. They're more likely to share a small piece of the nearest shelf instead of blocking leg room or eating up a huge percentage of your desk space.
The range of potential storage solutions are virtually endless with these smaller devices, and they're shaped only by your imagination and whatever advantages or limitations you might already have in your workspace. You can keep it behind your display, mount it to a wall, or place it anywhere else as long as it has room for ventilation.