There are different reasons. Here are a couple that come to my mind, on a practice swing with no ball.
1. There is no "result", so there will be no "disapproval" (internal or external). In other words, the individual will not be judging his own outcome or be judged by others. It is like a "free" swing. 2. There is no ball to "ring the bell*". Looking at a golf ball will subconsciously trigger previous "feelings" when you looked at a golf ball and then hit it and had an outcome (good or bad). This can produce "anticipatory fear" or "anxious feelings" if you're not confident in your ability. 3. Usually, the golfer is not in their "setup" position. This is usually a bent over, out of balance, arms reaching, etc. vs. standing relaxed, balanced, arms relaxed hanging and the club is usually a little, to a lot off the ground. This makes turning the shoulders and body very natural and easy. 4. There is no "point of contact". The practice swing is not aiming at anything. It is kind of a general area. My one comment on a practice swing is... if you're going to take one, it must be exactly how you intend to swing when the ball is in front of you... or it is doing more harm than good. * Pavlov's dog. A "conditioned response".
Move Less ... Get Good! DUPLESSISGOLF