At least for some of the people I play with, they enjoy the challenge of building a character like a puzzle, putting together different pieces to build something they find really cool. They like the numbers and the classes and the vast array of feats (3.5/PF is generally what our group plays).
I enjoy the roleplaying a bit more, and as such I tend to almost exclusively play casters, because getting encounters over as quick as possible means more time in a session for roleplaying. *shrugs*
Why would someone want to play DnD, PF, or a d20 variant and not be powerful? This game system is pretty unforgiving to people who are un-optimized for their level.
And in regards to building characters ahead of time and crafting how their future levels would look? Most people I know IRL have an idea of what they want to do in the future, so they're going to work toward that, and so that tends to also be shown in characters they build.