There are all sorts of cottage industries in the firearms business. My midwestern and southern relatives are extremely enthused about such things. Mostly it amounts to "customizing a stock weapon with aftermarket accessories" as far as I can tell - personalizing it and suiting it to your tastes and build. I'm not much of a gun guy - I can tell that this one has a better scope or a more comfortable grip, but the rest of it's kind of lost on me.

I think it's much the same sort of mindset as the folks who who like to customize their vehicles. In some cases, it's the same people, but if my relatives are any indication the overlap between those two circles isn't that high.