Think of it this way: everyone has a moment when they just want to take a break and goof off for a minute. It's normal, and a part of keeping a healthy and productive mind. Now consider how many other people will a person taking break affect. In a regular office, or even cubicle setting: probably just themselves, maybe someone else to whom they walk to. In an open-office plan? Everyone within earshot. With multiple people taking breaks at random, if you don't have someone with a strong personality telling everyone to shut up and get back to work, you can easily have half a day or more wasted just on talking bullshit. Individual cubicles or offices are a natural dampener for this.