He might be finding the correct balance point for the shot at hand. I don't do it to the level as the pros but if I am it's one of two things:
1: working out a hitch in my stroke/body pain to the point I can trust my stroke. Gotta get limbered up.
2: setting the balance point and where the cue rests in my shooting hand. It is only right in one spot for every shot. I want my stroke to feel exactly the same on all shots (or as close as it can be) and modifying where you're gripping is how you do it. Cue balance must feel right for a particular shot/distance from cb to rail etc.
Try not to get caught looking for imaginary balance points. There really is no fixed balance point on a cue. It changes from cue to cue, even the same model from the same manufacturer. There is a spot, as you mentioned, where the shot feels right but that is more a comfort zone than a fixed point. Unfortunately, you cant always be in that comfort zone so you have to play through, whether it feels like its in the perfect spot or not.
As for pre-shot motions of some kind, there are a ton of different routines that players develop over the years. As for that particular move, the only time I did that was when I had a tighter shirt or vest on and had to reach a little for a shot. I stretched like that to get the clothing a little looser. My own pre-shot routine was (and I say was because I tried for years before I broke this bad habit) was to swing my grip hand back and away from my body then in closer and forward for practice stokes while I was lining up a shot. Then I would get down over the shot and start stroking straight along the shot line. The two motions were in opposition of each other and that is not what you want to do. Pool is as much muscle memory as anything else. Depriving yourself of that is just hurting your game. Practice the way you play. And that goes back to the beginning with the pre-shot routines of different players. If its something they have been doing a long time, and its not hurting their game, they will just go with it and hope it messes with the other guys head a little.