This button:

can be clicked and toggled between 'On' and 'Off' states but it does not do anything.

It may look it may be something to do with the nearby "Grid Shown"/ "Grid Hidden" button or grid size element nearby - but it does not affect their behavior at all.
It may have a purpose unknown to me (in which case it should change its name) but much more likely it was just forgotten to be removed after some front-end change in the past.