Google is very into this whole cloud thing, so much so that it's offering unlimited storage of high-quality images and video as part of the new Photos. If you've got all your stuff online, do you really need it locally? The new Photos app doesn't think so, and it can help you save some space.
When your device storage starts to get full, Photos will now pop up a notification and Assistant card that offers to remove things. If you tap the button provided, it will find all the locally stored pictures and videos that are more than 30 days old and have been uploaded to the cloud.
All the images you delete are still accessible from that and other devices via the Photos app. It's something that other services like Dropbox Carousel have already been doing, but it's nice to have it built into Photos.