Click on that button, nothing happens.
Spam it, nothing.
Why is it there?

Ah, it's only usable when zoom level was changed, to reset it. But the zoom level can already be reset by clicking on the number in the middle (which is more intuitive to me).

The button could also be disabled while zoom level is at 100%, then a user changing the zoom, would see the button pop up, and associate it with the zoom level, making its purpose more clear.