"El Cap" is, simply put, the premiere big wall in the world. Sure, there are bigger faces or more challenge in Baffin Island, Patagonia, or other far-off places, but no other mountain face on the planet combines clean granite, generally good weather, easy accessibility, and enormous vertical like the monarch of Yosemite Valley. Big-wall aid climbing was born here, and today El Cap remains the center and main training gound for that subset of mountaineering.
However, for the peakbagger, El Cap is not even really a peak. Once on top of the cliffs, there is a very low-prominence bump and a forested ridge that leads up to the real mountains of the high Sierra. An easy hiking trail leads to the "summit" from the north, allowing almost anyone to easily claim that they "climbed" El Cap.
So El Capitan is perhaps the peak that most separates the peakbaggers from the traditional rock climbers. Peaks with real prominence like Half Dome or Mount Whitney or Mount Everest interest both types of mountaineers, even if they gravitate to different routes on the same peak. But the lack of a real summit for El Capitan leaves the "peak" entirely to the rock jocks who "routebag" their way up the Nose and other faces of what is really a huge cliff.