Well, it matters when you have to integrate with APIs and idiosyncrasies of third parties, and whatever draconian DRM they require. It’s obviously not impossible to add support for multiple backends, but from a development effort it definitely matters.

It’s not like streaming platforms are gingerly implementing open and common standards for music streaming. Winamp had support for .m3u and streaming back in ‘04, but we’ve moved on from that.

Sure, but presumably that is different from the actual value of the app itself—the frontend. It's just a matter of effort to connect the interface to the backend. Why you'd insist on tying the two together is anyone's guess (likely convenience).