Before starting to use spotify I had upwards of 200gb of mp3s (mostly badly sorted, tagged) that I used as a music library and downloaded huge amounts of music. These days I dont bother. Most of what I want is on spotify and I pay for a "premium" service with no ads. Was nice to reclaim those gigabytes ;)
I maintain my own music collection (it is sorted and tagged impeccably, btw). The disk space overhead (x2, backup, backup, backup!) is completely worth it to me to be able to listen to music how, when and where I like. I can buy any portable player I like, I can copy the music to any device, I don't need a constant connection to the internet to listen to my collection (which is good, as my "broadband" service is a turd), I can maintain the collection in a quality which is acceptable to me (i.e. not internet streaming quality)... There are probably other great reasons for maintaining my own collection but I just take this freedom for granted so am having trouble listing the other things I like about it.
A chunk of my collection is old stuff captured from my own vinyl collection, I would imagine there is much of it that is simply not available in Spotify's catalogue (I'd like to confirm this but blah blah my country etc.)