Take for example JUST google photos. When I say service, I mean a best in class experience across my phone, my kitchen screen-speaker, laptop, desktop, and living room tv. Google photos does not live up to its full potential from a phone alone.
A service gives me consistent access to the same data, from a variety of interfaces. Each interface is tailored to use cases for that device type. Desktop interfaces are going to be more powerful than tablet, tv, or kitchen screen-speaker.
It's the same reason I prefer Facebook Messenger to text messaging. Works great from any device I sign into.
(Not being able to set defaults is inexcusable at this point as a product, and borderline anticompetitive. Microsoft is playing an equally dangerous game with Edge.)