Nick Statt, the Verge:
Now, when you want to share a photo, you no longer have to create an entire album. You can send a one-off message to a friend, so long as they also have Google Photos installed, that contains a photo, just as you would on Instagram, Snapchat, SMS, or any other chat app. If you want to turn that thread into a conversation, you can both start chatting, as well as react to the photos with likes and share more. That way, the photos become a starting point for a conversation, much in the way photos have become just another form of communicating on social platforms.
Since Google Photos is now, effectively, a standalone messaging app in addition to being a place for your photo library, it brings the total count of apps made by the company which have some sort of chat functionality up to six.