The Google Photos website is getting a new backup feature. Each time a user uses the service, Google will review the folders selected by the user. This was reported by 9to5Google.
Traditionally, Google rolls out new features gradually. If it's available for you, click the Download button in the app drawer and select Backup folders. This will allow you to select the desired folders with photos and videos that will be automatically downloaded. After that, you'll need to give Google Photos permission to back them up.
It's similar to how Google Drive works, but it runs in the background. And in the case of Google Photos, users still have to go to photos.google.com from time to time.