You are not alone. I miss this all the time. Nevrtheless I can see at least 3 important ways in which multi-user design is useful:
1. Constraining a guest user so they won't access anything they shouldn't - when a guest comes to you and asks to use you PC and you want to be nice and let them.
2. Constraining resident server programs so you won't get pwned.
3. Constraining nonfree apps (their installers especially) so they won't put/remove/modify anything outside user directory (Windows apps love to do this).
1. Constraining a guest user so they won't access anything they shouldn't - when a guest comes to you and asks to use you PC and you want to be nice and let them.
2. Constraining resident server programs so you won't get pwned.
3. Constraining nonfree apps (their installers especially) so they won't put/remove/modify anything outside user directory (Windows apps love to do this).