1. Convenience (I never download or upload anything, and my playlists work and are automatically updated on the devices I care about)

2. Breadth of music (it doesn't have everything I want but it has a surprising amount of breadth in things I'd never care enough to amass deep collections in)

3. Easily accessible playlists from other people (I really appreciate the "This Is <band name>" playlists especially from Spotify)

4. Seeing what my friends are listening to all the time (I get a lot of new music this way)

Yeah, stuff goes away on the service. Yeah, certain less-popular genres are patchy and incompletely represented (and are we ever going to get Tool?). Yeah, the personal library limits are a bummer (although as someone who never uses this feature, I don't care myself). Yeah, the UI is terrible for certain things (classical music is especially bad, and I really hate that single-song repeat gets turned off in so many ways). Yeah, some of their clients are worse than others (why is the PS4 client's sound quality so bad and not changeable?) Yeah, there's no lossless versions of anything (I think).

And yet, for all that, Spotify has transformed my music listening, and I've been listening to a huge array of music for almost 25 years now. I listen to so many more new and interesting artists and songs on Spotify than I ever would have otherwise. I'll never go back, personally.