Lifting in gear requires a high degree of skill, and I think that's something a lot of people are attracted to. There's also a degree of danger. Bench pressing, say 1000 lbs in a shirt has to be an insane experience.
Would you ask a race car driver why he uses a car to artificially travel around a track faster? Would you ask him why he doesn't just sprint? Maybe you would, but you see where I'm going. Just like a racecar, powerlifting gear is a tool some people use to substantially alter a particular experience, to one that they find more favorable. I personally can't understand why someone would play soccer rather than basketball, but that's because I simply prefer basketball. We all want to experience the world in a slightly different way from our peers, and that, to me, is enough to explain why a person would enjoy geared lifting more than raw. To each his own