Help people find their way or manage traffic flow.
Put yourself in the shoes of your visitor. Is there anything more frustrating than driving around a business campus or wandering around a building that does not have proper signage?
Visitors, clients, deliveries, contractors; regardless of who it is that needs to find their way to you, you have to give them as much help as possible.
Good directional signage becomes a critical factor where you need to encourage the flow of traffic (vehicular or pedestrian) in a certain direction.