I'd add versatility to that list too. With DRM free files you can edit songs, remix individual tracks, create mashups of multiple songs, create your own mix-tapes/compilations, use them in your own videos and projects, convert them unto whatever formats you want etc.

They're also amazing for portability. Every phone, desktop PC, laptop and tablet I have can play MP3s, and while most can also run spotify, the same can't be said for all the TVs, DVD/blu-ray players, CD players, ebook readers, and game consoles that support MP3. It must have been an easy bullet point to add to a feature list because I see MP3 and other music formats supported in a huge number of devices. I've seen it supported in GPS navigation devices, alarm clocks, children's toys, workout equipment, and toothbrushes