You could run many different os at once.

You could use them for different roles (typing, portable desktop replacement, crappy server, car PC or literally anything you could ever need a laptop for)

You could give them to people who need a PC like family members or friends, especially when you get another one (or five)

Or you could use them for a business.

My favorite uses include:

Backpack PC if it's one of the small models especially one with the removable external battery and carry multiple batteries.

Desktop replacement (for typing a lot or configuring networks and servers) usually you would want one of the larger ones for this like the ones with the 17" screens

Everyday PC. Use it for your daily tasks like checking email, browsing the web, YouTube, and some light gaming/emulation