Good point CMHCoachNRef. Since I asked the question I heard it explained as your said. The keeper wants to be able to pick up the ball to waste even more time. So, in a situation where the keeper could easily catch the ball (or trap it with his chest as you said) he instead gently blocks it to the ground. Where, he assumes, he is able to waste time and still pick up the ball.

This does seem like a ballsy call for an AR or center to make, especially in a close game.