Hi,
I think that I agree with alot of what others have posted above, including that it may be partially in inborne trait of human nature (for at least some of us).
Growing up I had 5 sisters and I also noticed that with them, and my mother, they would do similar things with fabrics and patterns, frequently going to a fabric store buying different patterns that they weren't going to use yet, because they weren't sure what fabric to use, or buying fabric without a specific pattern to use with it.
I think that maybe deep down somewhere there may be a 'need' (for lack of a better word) in many people to 'have' stuff ready on hand for the day when you might get around to doing something with it.
On the other hand, I do know that I have bought a couple models in order to better understand how something was done on the full-scale object that the model represents. I have several unbuilt boxes of amphibious tracked vehicles in my basement specifically because of this, as well as several aircraft carrier models.
And finally, I think that there may also be a little bit of "I always wanted something when I was younger, but couldn't afford it then, but can now' going on as well. I know that when I saw the 1/72 scale Revell (ex-Matchbox) Flower Class Corvette at my local hobby store, that kicked in real quick eventhough I have no idea what I intend to do with it if I ever get around to building it. That model in particular is one I'm reluctant to start yet, because I'm not sure that my abilities in building right now are up to doing as good a job on it as I wish.
Similarly, I guess that there are also a couple models that I have bought because I did have one when I was younger and I wanted to try and rebuild a new copy.
So for me I guess maybe it might be part, wanting to have something, wanting to follow up on things I didn'y have a chance to do when I was younger, curiousity about stuff, and also maybe a little "wanting to relive stuff I did when I was younger too".
Pat