There are several ways to call a shot. The first way is part of follow-thru, to be visually aware of where your sights were when the trigger broke. If, for example, you are shooting and when the trigger breaks, you know that your sights were a little high and left of the target. So, you tell your buddy "I pulled that one, it will be high and left." When he looks through the scope, he sees a shot high and left.
Another way is to call the shot before you pull the trigger. Several years ago, a bunch of us were plinking on a pipeline right-of-way. About 200 yards out, I spotted a surveyor's stake and lined up on it. My buddy asked what I was shooting at. "See that surveyors stake, about 200 yards out? I'm going to put the bullet center of the stake, 4" from the top." Sure enough, I put the bullet where I called it.
How does this help your shooting? In the first example, knowing your sight picture intimately when the trigger breaks is certainly an aid to good shooting. Being able to tell a spotter where the bullet went is a tremendous aid to self-diagnosis of problems. If you know you pulled the shot, then you can know how to fix it.
In the second example, it's just bragging. But, it ain't bragging if you can actually do it.