They do more than just string and matching rackets. They also mold the handles for the players to their exact liking. Just look at Soderling's handle. It's a custom mold (foam?) handle with a lead pipe inserted inside it. There's a reason many of the top pros use their service. To be able to pick up their racket and not have to think about the differences between each one, I definitely think its a good investment for them.
Sure, there are other places that customize and match rackets. RPNY does customization for cheaper. Quality could be similar, i cant say. I also customize rackets for my friends (stringing, lead taping, silicone, extending rackets) but custom molding is something I cant do. So P1 has a certain expertise in this area.