My old mentor used to do this too, and I really never understood why. Since then, I have seen others do it too, occasionally. It looked more like a fidget than anything helpful. I once tried it just to see if this was some hidden technique I was missing out on, but I didn't notice anything particularly helpful - it didn't help me keep focused on the text or anything, and it distracted me personally.

I'm very curious what the benefit is, or at least the reason? From an apparent lizard brain :)