Every now and again, you might need to know the resolution of your Mac’s screen(s). As Benj Edwards makes note, on How-To Geek:
The resolution (in pixels by pixels) of your Mac’s display determines how much information you can fit on screen and how sharp the image is, so it’s important to know. If you run a full-screen game or application at native resolution, the image will look its best, so here’s how to find out what it is.
How to Find Your Mac’s Screen Resolution has all the gory details.