Sourcebooks, for me, I don't even need to play the game, I derive a lot of enjoyment of just immersing myself in the rules and fictional worlds they describe without even needing a structured narrative.
This is exactly why I have my collection. I've played D&D exactly twice but own around £4K worth of material...
I describe them to people who don't know what they are as basically being travel guides to places that don't exist.