Having a basement is a dangerous thing, and not just because of all the creepy crawly (real or imagined) things that might live down there.
It's dangerous in the same way that gmail is dangerous -- "unlimited" storage!
When you live in an apartment, you've got limited space. If you also happen to study the ways of IKEA, this space becomes a little less limited once you learn how to fit an entire life's worth of stuff in 127 sq ft, but still, there's only so much space, and most of this space is your actual living space, so you don't necessarily want it cluttered.*
So, every once in a while, you go through your stuff and realize that you've got too much. Something's gotta go. Thus the Salvation Army gets a nice gently-used donation or your second cousin who just started college and needs some hand-me-down furniture gets that random chair you've had since you were in college and thought it was the best thing ever. You free up a little space, and the natural balance of your apartment returns.
But in a house with a basement, you've suddenly got this mysterious space available for things like this. It's not exactly used as your living space, so you don't have to regularly see the stuff down there. It's wide open and unfinished, so there's plenty of room for stacking. It's perfect for those things that you just don't really want to deal with right now. I know, I'll stick a box of stuff in the basement and figure it out later!
Except later never really comes. You're too tired to descend all those stairs just to sort through things you probably don't really need. Or you're saving it for when you have kids, thinking (irrationally, of course) that they might want to play with that stuffed animal you had growing up that no longer has any fur and is missing its left eye. Or maybe out of sight is really out of mind, and once it can't be seen, you just don't have to deal with it anymore.
The moral of the story? Don't have a basement. Or if you do, don't be fooled by its seemingly innocent ways. It all starts with just one box...
*Unless you're a hoarder and like that sort of thing.