With timed exclusives they can also capture some amount of revenue from conversion of free trial users into "forgot to unsubscribe" users.
They got me with Coloring Book a few months ago. It wasn't available elsewhere, so I signed up for the free trial. Well before the free trial was over Coloring Book appeared on Spotify (my preferred platform). I prompt forgot about Apple Music until I saw a bill in my inbox. I then couldn't figure out how to unsub without googling the process, it's certainly not readily available as an option in-app (although that just might be the way Apple sometimes tends to obfuscate through streamlining).