Isn't there anything you already know but wouldn't like to forget?

Yeah, that's pretty much the problem. Not really. I.e. there are stuff I know that would be inconvenient to forget, because I use this knowledge every day. But since I already use it every day, SR seems unnecessary.

Things I don't use every day are not essential - the cost of looking them up is minuscule since it happens rarely.

I suppose a plausible use case would be birth dates of family members, if I didn't have google calendar to remind me when needed.

Edit: another use case that comes to mind would be names. I'm pretty bad with names (though I've recently begun to suspect that probably I'm as bad with remembering names as anyone else, I just fail to pay attention when people introduce themselves). But asking to take someone's picture 'so that I can put it on a flashcard' seems awkward. Facebook to the rescue, I guess?

(though I don't really meet that many people, so again - possibly not worth the effort in maintaining such a system)