If you're reacting to click events, you might want to know the coordinates of where you're clicking. I mostly use this for click and drag stuff since you can get the delta between events and update position of the thing being dragged.
As for why they're checking for coordinates instead of checking for event.type is beyond me. Still I appreciate the write up, it is a good puzzle and relatable to come across code you didn't write and ask, why is it important that the click coordinate is nonzero? Why can't we just check that event.target is the button we want to activate? Why are we using JavaScript at all when a details/summary tag would do the same job?
As for why they're checking for coordinates instead of checking for event.type is beyond me. Still I appreciate the write up, it is a good puzzle and relatable to come across code you didn't write and ask, why is it important that the click coordinate is nonzero? Why can't we just check that event.target is the button we want to activate? Why are we using JavaScript at all when a details/summary tag would do the same job?