Choking down on the club.

I don't know if any of you have noticed how some of the PGA players choke down on their clubs. For instance Steve Lowery, when he won at Pebble Beach earlier this season I noticed that he choked well down on many of his shots. Yesterday when Anthony Kim turned in that excellent round during the Wachovia he also choked well down the grip, on one drive in particular he was a couple of inches down the grip with the driver and smoked it perfectly 322 yards.
I'm just curious as to what their train of thought is by choking down so much ??

My impression is that it gives them more control. They're effectively playing a shorter club.

It is usually done to control distant, gripping down shortens the club and reduces distant and spin. Less club head speed = less spin.
Here are some other reasons for gripping down:

1). If you are playing a shot into the wind and need a lower flight, gripping down helps reduce spin, along with playing the ball back in the stance.
2). takes yardage off a club, normal 7 iron goes 150, for every inch you grip down takes around 5-10 yds off.
3). If the ball is above your feet too offset the difference too make it a more normal setup, won't have to swing as flat.
4) chipping for control purposes, since you are not interested in distance. I never chip without gripping down several inches.