there are some reasons, good or not is up to you.
1. when you jam on thrust what happens on a 737? the nose goes up...underwing engines will push nose up ...cutting power brings nose down
2. Airplanes get bent up in real life...wing leading edges get roughed up in real life...if you cut power too soon the drage may make the plane slow down a bit faster than the ''ideal'' brand new plane or simluator.
3. Here is a real help...if you and the engines are spooled up just at touchdown, they will be more available for reverse thrust...in other words jam up the throttles, engines spool up and retard them allowing reverse thrust selection and the reverse thrust is available more quickly as the engines are still spooled up
4. It is possible the pilot is adding the thrust because he has perceived a high sink rate developing and you haven't
6. notice I missed five?
5. I've flown botht the DC9 and 737. DC9 is my favorite. and a quick burst of power is used when a high sink rate develops near the end, usually if the airfield is blanked by trees, cutting off the wind or if there is windshear for other reasons. judging sink rate can be a very ''seat of the pants'' thing.
so ask yourself if the pilot is doing it at fields where the trees blank the wind