I put Glock into much the same category as the Remington 700, AR-15's, Ruger 10/.22's, and the 350 Chevy. Because of their popularity, they've developed a huge aftermarket. People who own them want to modify them in a way that sets them apart from all of the rest out there.
The difference is I doubt any of the modifications improve performance the way it does on the others I've mentioned. In fact, I think several of the Glock modifications out there can actually degrade performance. Speaking only for myself, it seems every time I'm at the range and I see a problematic Glock, it is most always one that has been modified in some way. Of course cosmetic changes like stippling and sight changes aren't going to effect performance. It's only when people start messing with the internals. I for one have had it proven to me Glock got it right the first time. That's why all 6 of mine are box stock.