"Customizing", "modeling", "making stuff"... whatever you want to call it. (I'll call it customizing to keep it simple, but it really represents a continuum from simple modifications to building from scratch.) If you're someone who does this, you already know the attraction. If you don't, then you might wonder why or how.
Hopefully, the web site shows some of the "how". The "why" is easy: Without this alternative, your choices are limited by what someone else produces and the availability of those items. If you make your own:
For the casual collector and fan of a particular series or genre, customizing is an attractive endeavor. It allows you to fill gaps between what the national manufacturers produce. If Hasbro/Kenner doesn't create an particular obscure character from the Star Wars Trilogy, then a customizer can. In addition, it's possible to create something far more interesting than something mass-produced in a factory. Why? Because you care, are willing to spend the time and use techniques and materials which are not cost-effective in a production environment.
If it's an item that's not mass-marketed, you can either pay the labor costs to someone else, or you can consider your own time as free and make it yourself. If you're satisfied with your results and the materials costs weren't inflated by the wastage of trial and error, then you can save a bundle of money. To me, it's a sort of "soft" justification, because I do plenty of the wastage thing. If I'm determined enough to make something, money gets spent blindly... (You can always use the leftover stuff for something else, right?) Non-production customizing doesn't place too much value on efficiency. And it's hard to place value on something which isn't available elsewhere.
As for gratification-- yes, if you love to work with your hands and create things, you already know this. You know that it requires persistence and determination. It takes the quality of refusing to accept failure-- even though you may feel that what you're working on is going south on you, you know that you can crumble it up and get a fresh start, and pretend that you intended to do that! Failure (of course, it's temporary!) is a great teacher...
At a given moment, each customizer checks in somewhere along the scale which balances service to fandom/the love of collecting and the love of creating/working with his hands. When the balance shifts to the craft side, loyalty to a particular fandom becomes less important, source material is evaluated in terms of its design and inspirational value, and pride in craftsmanship become paramount. In other words, for the craftsman, what you make isn't as important as the process of making it. When you're done, your creation stands as a visible reminder of the lessons learned in making it. This is a somewhat more intense gratification than glancing at a dust-covered collectible you paid good money to acquire. And that's one of the things that makes it really hard to part with a piece that you've created.
I'll be completely honest and tell you that I've come to "un-appreciate" die-hard fanatical fandom. I've seen fandom carried to extremes-- people who feel the need to denigrate Star Trek because they love Star Wars (and vice-versa)... People who become obsessive about the minute details of a movie series and slavishly seek to fill in the details (according to "canon") to cobble together a consistent "world"...(and argue about it!) It's an imaginary world, though, and one which probably goes way beyond what the original creators ever intended. Whatever turns you on: Using your imagination is fun. Unfortunately, it can be taken too seriously, and manifests itself in really nasty and unpleasant things being said, with genuine feeling. It's as if the imaginary stuff has become internalized, and as real as the real world. There's a Nazi-esque vibe to this.
My feelings on this may seem arrogant... I'm glad that talented people have created these wonderful things, and I'm more than willing to feed off of their inspiration. However, I don't view them as gods and and am unwilling to transform their works into a personal religion. In other words, I'm not much of a Hero-worshipper. To me, heroes are mere mortals with superior accomplishments and qualities we should emulate, to improve ourselves. Self-improvement doesn't step on anyone's toes, and it's the most obvious and natural destiny we have as individuals. I believe that's the reason we're here-- otherwise we're just pooping machines which like to collect kewl stuff.