Google Photos is an excellent tool for storing and viewing your photos, but it has a number of other useful features. Most practical for most people is that it can be used to free up precious storage space on your phone by removing photos that have already been backed up to the cloud.
To do this, tap the hamburger button (three short horizontal lines) on the top left, or swipe right from the left edge of the screen, to open the menu screen. From this screen, tap Free Up Space. You’ll see a notification that will tell you how many photos will be removed from your phone and how much storage that accounts for. Just confirm by pressing the Free Up button and you’re done.