I have an "empty" email folder. Nothing ever goes into it. Once I'm done checking my dozen email inboxes, I switch back to the "empty" folder and it gives me the feeling of "nothing in my inbox at the moment".

Clicking “Archive” also has the same effect + you can easily finds things while not being at the computer.

It just removes the clutter from view. It’s not the same thing.

I’m using this system as well.

I don’t keep letters in my mailbox. I don’t see why i should keep emails either.

This analogy isn’t convincing for me. My letter box and my email inbox both have a finite amount of space, but I can (effectively) infinitely extend my email inbox either “online” or by archiving it someplace.

I cannot achieve this with my letter box.

That assumes large volumes of important mail. Even without that though, an email inbox is a natural and “first class” place to store an email, whereas a letter box is not designed to be effective at any sort of reliable storage.