Tony Stark is a nerd. Well, technically a geek because nerds are experts on things that aren't real (yet). Geeks are experts on things that already exist. Math, engineering, archaeology, psychology, forensic accounting... lasers.
Since Tony Stark is a geek, he builds an "advanced prosthesis device" using a technology that only a handful of people in the world even know about.
Since Tony Stark is a nerd, he integrates particle beam weapons into the palms of his suit. Sure, repulsors were awfully useful for flying, but what nerd doesn't want to be able to shoot energy out of their hands?
Since Tony Stark is a geek and a nerd, he adds lasers to the suit. He isn't even sure if they'll ever be useful. They certainly aren't practical, having one use and all. But they're fucking lasers.
He simply cannot resist.