Looking busy can sometimes be a defense mechanism for protecting your own space and peace of mind. When your cow-orkers, friends and family see you Very Busy, they'll be less likely to interrupt you all the time with irrelevant bullshit.

In fact, this works extremely well, because people usually respect the fact that someone is working. On the other hand, when they see you not working, they think it gives them the right to acquire your time.

Sure, a socially savvy person could probably handle all of this by simply talking to others and asserting the boundaries. But for the rest of us, this simple trick of looking busy can help reduce the amount of human interaction to a remotely healthy level (and for many people, myself included, even the act of assertive rejection is hugely mentally taxing).