There are a lot of applications, not to mention the Google-specific ones, that require Google Play Services to run correctly on your Android device. Without Google Play Services on your device, these apps are virtually useless. People who are working for more autonomy from Google do not like this one bit, and this new app called microG GmsCore might just give them that.
We have to remember that this app developed by XDA member “MaR-V-iN” is still in alpha stage, and would therefore have the capability to possibly brick your device, so we recommend using it with extreme caution, if ever you do plan to use it. This app is definitely for those people who do not want to use Google Play Services on their phones.
The microG GmsCore app is a Free/Libre Open Source Software (FLOSS) where you can install Google apps and still have them function without installing the required Play Services app. Developers might need this kind of isolation, and so too people who create custom ROMs.
If you are in that line of work or hobby, you can check out the official XDA thread for the app here. We don’t have to remind you again that you need to keep usable backups of your phone data when dealing with apps like this, but there, we just reminded you again.