There are a handful of Google services that virtually everyone uses, and Google Photos has managed to squeeze into that select group. Over the two years it's been out, Photos has accumulated over 500 million users as of May 2017. Although the service still isn't perfect, it's getting there. The latest update enables watched videos to be cached so that they won't use up more data when replayed.
It's a simple premise - caching the video will prevent your device from consuming more data should you rewatch it later on. It's a pretty smart idea, as in my experience, I've rewatched videos uploaded on Photos multiple times. The only issue I could see is if your device doesn't have much storage, in which case you'd have to clear your cache more often.
The update is live now on the Play Store, but if you're not seeing it, you can grab the Google Photos APK from APK Mirror. Let us know what you think of this change in the comments below.