1. The catalog is comprehensive. I listen to far more music than I could afford to own. 2. There are no advertisements in the paid service. 3. Their music discovery algorithm is excellent.
I also appreciate the yearly statistics, and how they continue to add value for me. Podcasts and eBooks being added to the platform was cool. I like to make "taste combo" playlists with friends. Really one of the only companies I genuinely feel deserves my money.