Simply by selecting Enabled, you've allowed your user to click on the image to view a full-size version, with the rest of the page dimmed in the background. We call this the "Gallery View."
Also, as the name suggests, it's the easiest way to make a quick slideshow on your page. If more than one image on the page has this function enabled, left and right arrows will display in the Slideshow for navigation, and a "Start / Stop Slideshow" button will display below the image.
Here's a quick example of how you can use the Pop-Up Slideshow on your page. Simply format the thumbnails the way you like, then in the edit dialog choose Enable.