GmsCore is helpful for Android users who want to stay away from Google (even when application developers don't!). For example, it makes it possible to use Signal without depending on Google Cloud Messaging. (This is also helpful when Google stuff may not necessarily be available, such as on Amazon devices.)

https://news.ycombinator.com/item?id=10665520

https://o9i.de/2015/10/23/howto-gmscore.html