Why do you have a colossal...

[Mighty Halfling]

I've been playing since the 1980s, and love to look through the old stuff. From the flavor text to the art, I just kind of dig it.
Of course I don't just have D&D stuff. I have a fab collection of D6 and D20 Star Wars books, lots of other D6 System books and a scattered array of other RPGs.
It all adds up to a 4 foot shelf of favorite books, two or three tubs of additional books, a backpack crammed full for game nights, two buckets of official miniatures, a hefty set of paints, an overfull dice bin, a box of magazines and probably some scattered items throughout the rest of the house.

But why do I have it all? What's the real reason I love it all so much? Because when I play or plot as a game master I'm creating something new.
The "act of creation" is the foundation on which all of my hobbies are built.

 

[UselessTriviaMan]

Like most, I'm unwilling to throw out/give away my old books. However, I do like to mine the old stuff for tasty tidbits I can reuse, re-skin, or blatantly steal for more current campaigns/scenarios.

I don't consider myself a completist, but I know a good deal when I see it. When the DDM eBay boom hit, I started scooping up all the pre-painted plastic minis I could get in my grubby lil' paws. I have five tackle boxes full of minis, and more metal minis in display stands/cases.

I am on the verge of purging some of my collection though; I have game systems that I'm never going to play, and really don't need to keep. (This will also greatly please my wife.)

 

[Zhaleskra]

Because during the '90s, I suffered from "shiny new toy syndrome", often buying a book for any game I'd played a single session of. In the late '90s, after looking at my Planescape collection in particular, I realized I was relying too much on the imaginations of others. Some of my books have been sold off, and while there are some lines I want to follow, I want to keep it to a maximum of 4, and then only of games I actually intend to play or run.

 

[Voadam]

I've been gaming since '81 and have accumulated a lot. While I generally read novels once and then donate them to a library, I keep game books and keep acquiring ones on topics I'm interested in. I consider game books to be reference books and I like having a lot to reference as desired. I don't know what my gaming future will hold in a couple of years, whether I will be specifically interested in referencing dragon ideas, or aberrations, or undead, or psionics, or Oathbound, or Conan, or Changeling the Lost, or Ars Magica, or Freeport, whether I will want a jungle adventure, or an urban mystery, or a mega-dungeon, or a pre-done adventure path but I can dive in and have plenty of options as desired and things come up. It is my hobby and I budget myself a monthly amount to get stuff I'm interested in, usually PDFs now but my amazon wish list is filled with game books as well. I semi-regularly read books from my collection, either all the way through or piecemeal for specific researching for games or general interest. I have more than I could use or read in a lifetime and don't have a burning desire for more, but I do have a moderate desire for more and the means to reasonably afford to keep getting more so I keep acquiring more things that interest and appeal to me.

I also have slight obsessive and pack rat tendencies so that factors in a little as well.