My good faith response to your good faith question: because having a DRM-free copy on your own server or hard drive is the only way to be sure you will be able to play it tomorrow.

Streaming services are a complex collection of licensing deals that are by design temporary. You may not hear beforehand when your favorite artist's label's parent company's conglomerate's CEO decides to pull their content because they're going to start their own streaming service, or another service gave them a lucrative exclusive deal.

And while you're never going to have a hard time finding Taylor Swift, that one 70s esoteric album may become instantly impossible to find once it drops off a streamer.

In the end there are no promises with a streaming service. On the other hand, you put in a small amount of work to grab MP3s or FLACs, set up your own Plex server (or Emby, etc), and you're good for pretty much forever.

Similarly, support artists by buying their direct merch, going to shows, and so on, but they are barely seeing any Spotify money. Between Spotify and the labels, they are cleaning the plate and artists are getting whatever crumbs fall off the table (unless you're Taylor Swift or another global artist).