But it’s still weird to put something that you still need into the trash can on your desktop computer.

Like, if someone at an office physically put a printed document into the trash can next to their desk, and then they started screaming the next day because the cleaning staff had emptied the trash can of “important documents”. Most people would agree that that would be an insane reaction.

So why people decide to intentionally put important documents in the trash can on the computer I don’t understand.

IMO the purpose of the trash can on the computer is to give you a chance to undo throwing it away, just like in real life. Instead of putting it straight into the shredder irl. But to expect things to stay in the trash for more than a little bit, that’s bonkers.

Only because you (should) expect (through experience/onboarding etc.) that that will happen. IMO it's a bit more like leaving a pile on your desk that looks like scrap, then being annoyed that someone cleared it without telling you or when you asked them to help with something else, and it actually wasn't scrap, or at least you hadn't yet determined that it certainly was.

And browser history doesn't even look like scrap, it's just a useful thing that exists and many people never clear it. Myself included, no reason to.