It's also a marker of personal respect and status. If you walk into someone's office, like a lawyer or a doctor or a professor, do you sneak up behind them? No. They're facing the door. The desk is between you and them. Behind them are bookshelves or a panoramic view of the local freeway or diplomas framed and mounted on the wall, and you're face to face. Even receptionists get this courtesy. It's comparatively kind of degrading to work in an environment where as soon as someone enters your workspace they're looking over your shoulder.