To allow yourself a momentary disconnect. To watch the world out of the window, or the people around you. To take an interest in the weather and season. To give your mind a chance to digest what you mean to do or what you have done.
I cherish my time travelling from one place to another, even if that is just a walk to the shops, a trip on the bus or ride on the Underground.
I also like to have 100% focused work time and space. It's hard to find that focus when you're willing to mix work with everything.