I can be almost be certain that all of your Pi's and PC's have multiple users, many of them. Who might they be?Most of my PI's and PC's don't need multi user OS's, so why have complex OS's I don't understand ?
1) Yourself of course.
2) Yourself masquerading as the root/admin user.
3) In the case of Windows, Microsoft. They like to control your updates and spy on you.
4) Arguably all the authors of all the software you have installed on the machine.
6) All the visitors to any pages you have served up by a web server that uses PHP or some such.
7) All the authors of all the malware that is around looking to get into your machine via various network connections or whatever other interface.
Security aside it's useful to have 2), that can save you from shooting yourself in the foot. Like for example accidentally deleting the file system when logged in a yourself. Or perhaps some buggy program you are developing does something bad like that.
Security aside, it's useful to have tasks running in the background, they are likely to be the main purpose of the machine, whilst you only actually log in occasionally.
Then there is security. See 4), 5), 6), 7) and no doubt many others I have neglected.
Of course 3) is a good argument for a single user operating system