1. Artists / labels can pull releases from streaming services, so an album may completely disappear from one day to another. They won't come to your house and rip the CD out of your CD player.
2. They can also replace one version of a song for another, like a remastered version. Especially remasters from the "loudness war" era are often worse in quality than original releases. On a streaming service, you often don't have the choice of which version you can listen to (and if you have the choice, point 1 is still a danger). This is also a problem with songs that contain explicit lyrics. The streaming version may force you to listen to the version with those lyrics removed (e.g. US release vs. EU release). Again, you are not affected by this if you have the physical media or a copy on your computer.
Having a copy of the music that noone can take away from you is important. Tons of music keeps disappearing from streaming services. This is not just a made-up scenario. Yes, technically speaking you only "own" a license to the music. The important thing is that this license is practically irrevocable.
