jdrakeh
Front Range Warlock
For me, in most cases, the draw is getting a fairly comprehensive overview of the IP under one cover (or in one box set). All of the various treatments of Thieves World, for instance, satisfy this quality. The two editions of the Discworld RPG (and I'm assuming the forthcoming new edition), as well. I mean, I could attempt to read all of the very many novels and parse all of the information therein into gameable fiction, but that's a lot of work. If others can do it for me, I'll let them do it.