This is a specific case of the general phenomenon. Lots of people walk and move around while doing something. It’s just how the brain works.
Try this- if you typically meet somebody for coffee and conversation, try having a conversation while walking together, aimlessly. You’ll find that the conversation actually flows better. There’s less “blockage”, more a sense of common purpose. It’s easier to resolve arguments this way. (Both Steve Jobs and Mark Zuckerberg used to hire people by taking them on walks. Source: http://www.businessinsider.com/mark-zuckerberg-walk-in-the-woods-2011-7)
Nassim Taleb advocates this.
Here’s an interesting corollary- have you ever spent hours reading a book, and found yourself fidgeting, rolling over, climbing all over? Here’s a depiction of the phenomenon I’m describing: http://static.lolyard.com/lol/reading-positions.jpg
The point is- humans were never designed to sit still. The real interesting question is how come we sit still as much as we do. Our brain isn’t compartmentalized into “move” and “think” and “communicate”- they overlap significantly. Friendships are often built through shared action- sports buddies, gym buddies, etc.
The brain is very interconnected, behaviour is very interconnected, movement is part of how we express ourselves. It’s weirder if we sit completely still.
Additional thought/reminder: Lifting helps to break through limiting beliefs by demonstrating to you that you can lift more tomorrow than you can lift today.