They're releasing an android app to compete with Spotify on the family accounts, it's quite likely that at least one member of a family doesn't own an iPhone and uses an android phone. These families would use Spotify (or any other cross platform service) instead of apple music.

Worth noting that Apple releasing Apple Music on Android is the exception to the rule - what other 1st-party Apple software/apps are available on Android?

Also, Apple Music on Android isn't even released yet :-/