As I am a newbie, I use google-play-services for Admob and to put maps in my application, but other things I can add to my application with Google play services?
Google Play Services is a large library with many features. It is installed on most phones. This is a way for Google to update features without updating the OS (which they can't do themselves).
Many libraries depend on Google Play Services. Examples: all Firebase libraries, FusedLocationProvider, NotificationListener and others.