Hot corners are shortcuts for your mouse. Just throw your mouse cursor to the corner of the screen (the easiest place to hit) and you can instantly lock the screen, start a screensaver, show the desktop, show all windows and more.
If you use your Mac with a standard two-button PC mouse instead of Apple’s Magic Trackpad or Magic Mouse, hot corners can replace the multitouch gestures that you miss out on.
Hot corners area really quick and easy way to help navigate your Mac, and I recommend you turn them on and use them.