I think what Spotify does right is that it actually feels more like a straight-up legit music player. I think it was the first one that really made it feel simply like your phone had 35 million high-quality songs on it, and Spotify was just playing them. The free tier is also cool, as you can listen to everything, just with an ad every 30 minutes and a limited amount of song skips allowed when going through a playlist.
Listening to the music on Youtube just feels wrong somehow, as it doesn't feel like it's made for music. The convenience isn't there, and I'm just somehow used to the fact that going to Youtube for music is going for those obscure, low-quality community uploads of niche songs. For proper listening, it's like buying furniture at the grocery store kind of feel.