Rain Designs iLap has worked well for me; I have one for the 17" MBP and one for a 15" MBP. Right now my new 13" rMBP is sitting on the 15" iLap until I can get one that is the proper size for this machine.
Why a cooling pad or stand? The earlier MBPs used to get pretty darned hot and very uncomfortable on my lap. Also, the iLap looks nice on the desk and protects the bottom of the rMBP from rough or possibly dirty surfaces. It also protects the machine if any liquid or food is nearby -- reduces the possibility of the machine sitting in a puddle of spilled liquid. The iLap works great just about anywhere -- on a bed, on the sofa, or on a table or desk.
For years I also have had something really nifty for travel -- APC used to make this thing that folds up like a folding fan -- when opened, it creates a handy 4-legged surface upon which to rest a laptop. I don't think they make them any more, or at least I didn't see anything when I checked online.
As far as actual need for cooling, per se, I don't think it hurts to have more air flow around any computer or electronic device. My Mac Mini, for instance, I elevate on an aluminum trivet to create more air flow around and under it, and ditto for my Airport Extreme.