I have to say, I've never done orienteering - I believe it's a suburban form of wilderness trekking/TA exercise. So, bear with me, if you have a sense of direction, and sense of time, why is a compass an essential unless the mist has come down ? Why is a map necessary if you're already familiar with your surroundings ? Carry both (and telephone, and maybe GPS) for the sake of safety - but how often does the experienced person actually need to use any of them ?