Manager's responsibility is to talk to people. Often on the phone. People around you can hear everything you say. You have two options:

- Spend half of your day in conference rooms;

- Stop giving a single fuck about the privacy, and 'excel in transparency' (or whatever corporate bullshit is applicable).

Same reason as cubicles for the paeans. Status.

But also, if there's a manager in the middle of the bullpen, people tend to clam up. I experienced this. Managers can have a very hard time handling the typical heated and frank discussions that can arise among engineers. At one time, we had the project manager in our cube maze along with the engineers. If he sensed that two engineers disagreed with one another, or that they were discussing a problem, he would step in to manage it. The obvious outcome was that the engineers found another place to hold our discussions.