There is a section of text from the main website that I think sums it up:
"Think of Spotify as your new music collection. Your library. Only this time your collection is vast: millions of tracks and counting. Spotify comes in all shapes and sizes, available for your PC, Mac, home audio system and mobile phone. Wherever you go, your music follows you.
And because the music plays live, there’s no need to wait for downloads and no big dent in your hard drive."