Google Photos is the latest Google service to get a Progressive Web App version. It’s following in the footsteps of Google Maps Go, which got its own lightweight web app last December, via Android Police.
To set up the Photos Progressive Web App, go to the Google Photos site where you can now add it to your home screen on Android or desktop (assuming you're using Chrome, of course).
Right now, there isn’t much new in the Progressive Web App version of Photos. There’s no support yet for notifications or offline support, so you’re mostly just getting a glorified link to the Google Photos site. But it’s still early days, and it’s easy to imagine that Google will update the Photos web app with more features that could make it a viable, lightweight alternative to the full-fledged Play Store Google Photos app in the future.