Seems to me there are two primary appeals to hyper-detailed character design systems, wherein lots and lots of traits are spelled out with as many options as possible:

First, they appeal to the “hobbyist” character designer, that is, they make character “modeling” or design as much a hobby activity as building other sorts of models (model trains, ships in bottles, etc.) and sharing them with fellow hobbyists. Character design itself becomes a kind of stand-alone or sub-hobby. Note, I think this is different from set-ups where character design is treated as a sub-game, such as Evil Hat’s Spirit of the Century wherein creating characters can be a fun shared social experience.

Second, they provide a kind of almost “vicarious” game play: reading in detail about what a cool trait can do is almost as good as seeing it in action. Multiply that by lots and lots of cool traits, many more than you’d ever see in action in a typical RPG session or even campaign, and you’ve got a fun experience unto itself, just imagining all that cool stuff in play. This can help to spark the imagination, leading readers to figure out what they really want to see in play, and feeding back into the modeling aspects of character design (”That sounds really cool, I wonder how it works? I’ll have to design a character with that and try it out.”)

The interesting thing about these aspects is they also make the system appealing as a purely intellectual exercise. That is, you don’t even have to play the game to enjoy it! For some a significant portion of the fun may be in the “gearhead” aspects of design, creation, and study of all the various aspects of those sub-systems. This may support the continuation of hobbyist communities for certain games even when active game-play seems relatively limited.