It could be younger kids or elderly people, or anyone else who doesn't have a phone or tablet, and you want them to be able to access the gallery and check some pics or videos when you're not around to cast the content to the TV. The second reason is to allow someone who doesn't have a smartphone to browse your gallery on the big screen. For most other people who don't need the automatic backup to kick in each time they take a screenshot, there's always a slower manual way to send screenshots to other devices - uploading from Solid Explorer to Google Drive, or using Send Files to TV are two methods I've often used. This is clearly something only tech writers, gamers, app developers, or tutorial writers would find useful. The first one is to set up automatic backups and sync your screenshots to other devices. As far as I'm concerned, there are only two valid reasons to get Google Photos running on your Android TV. I could talk about how more convenient it is to browse your gallery directly on the TV instead of looking down at the phone then up at the big screen, but really that's a tiny nitpick.

Google pics app