It is part of my setup adjustments based on experience. It is a form of "fine tuning". I know the shorter the length of the shaft = shorter radius = less distance = more control (in general). What that all means to me ... if I choke down, I can be more aggressive with a smaller radius. I feel "less can go wrong".
I've also practiced with feel for distance and ball result (after it lands), at different choke down lengths. All golfer's would probably benefit if they learned to choke down (as an option). Another reason is when you're in between clubs. Let's say you need a bit more than your 8 iron will cover. You can take a 7 iron and choke down the shaft (maybe 1"), and get the shaft length of the 8 iron, but with the loft of the 7 ... result = a bit more length. You must practice these to see how the results vary. These are advanced "fine tuning" techniques.
Move Less ... Get Good! DUPLESSISGOLF