@trinsec I've never had a middle button. What is it for?
@tonwood It usually does the job of a ctrl+click. In my case it'd be easily opening a link in a new tab, and I use that a lot.
In games it has various purposes. The most common one being able to pan in a map.
You sure you don't have a middle button? Usually there's a scroll wheel, and you can use it as a button too by pressing on it.
@trinsec my Logitech Marble Mouse (trackball) has left/right buttons plus two tiny ones whose function(s) I have never learned; possibly in MacOS they do nothing.