We leave drives A and B reserved in honor and remembrance of floppy disks. Think of it as reserving a famous sports player's number. Those things could hold so much data that we still use a floppy disk as the icon for saving things today.
You can also assign numbers (and I believe even entire strings with a custom kernel driver) but Explorer doesn’t recognise them, so only really useful if you’re desperate for more mount points and don’t just want to put them within other folders.
Or you just want to be silly and make your OneDrive the 1:\ drive!