Google Play Services (GPS), the Trojan app
Isn't this the every growing closed "app" into which Google is embedding ever larger chunks of Android functionality, rather like a (closed) SystemD for Android? No doubt at some point it will grow to the point where it *is* effectively Android, at which point Google could dispense with the Linux kernel. (Last time I checked, manufacturers are required to include the app to as a condition of using the Android name.)
Ostensibly this use of the app was to allow patching of devices that were not receiving core Android updates. In practice numerous applications that refuse to run if GPS (the app) is not running, including many official apps such as Google's Gmail and Calendar apps, even though it is not clear why they should break (GMail actually displays emails etc. before popping up a dialogue insisting that the GPS app be re-enabled), or why they cannot fall back onto "open" Android APIs.
It's interesting that Google Maps runs fine without GPS (the app), even though it ostensibly provides supplementary high resolution location services. Looking at the services provided, it seems that GPS (the app) can provide location information even if GPS (the service) is disabled, using WiFi location data. Perhaps the name of the app is no coincidence if both it and Google Maps are phoning home with tracking information.
There are many alternative apps that don't rely on GPS (the app) such as K9, Etar, etc. (and you can use Osmand instead of Maps, and an app to update AGPS data, such as SatStat) but it is rather annoying, and troubling, that many do, including Signal (ironically in the interests of "security", and for push messaging, even though ChatSecure seems to function just fine without it, including deferred delivery for intermittent connections).
[You can run a phone without Google Play Services (for now), but fun may follow if you try to disable to the Google Search App: that breaks the default home screen, leaving you with what appears to be an unbootable brick, until you phone yourself, and re-enable the search app while answering the call (not the most obvious or easy-to-find solution), and then find a launcher that allows Google Search to be disabled.]