I have found Playtesting to become a vital tool in making good development decisions at different times in the project from concept to finishing. It is an additional data point that helps prove out and support ideas and assumptions we have created. It gives me additional information and insight I didn’t have before which is useful both for internal and external discussions around the game.
I worked with Steve on a title where we had many thorny issues that only playtesting could aid us to make headway on.
He has a real clarity of understanding of how to translate your questions and needs into practical unbiased tests and programs to get the understanding you seek.