Apple Users

I can think of a few reasons ....

1) Cats (or children). You can (and should) set it up to need a password to unlock. Keeps kids and pets from random keyboard entry.
2) You can still hook up old monitors to your new laptop. They might still need a screen saver to save on phosphor.
3) Clock. You can have the current time up and use it as a wall clock when you're near but not using the computer.
4) Snoops. To keep spies and such from reading your screen if you're called away and forgot to sleep your system.
5) Theft. If you have full disk encryption enabled, having your screen locked when you turn your back for a minute is a good thing. If you don't have full-disk encryption set, then it doesn't matter, you're letting anyone who steals your computer have full access to its contents.

But yeah, instead of screen savers I tend to hot-spot a corner to make it turn off the display. And I've got it set to require a password to "wake up" if it's been more than 5 seconds.