In an ideal world, sure. But when shooting out doors, often there are too many other sounds. Maybe that dock is near a highway. Maybe there’s a festival nearby. Maybe the director is yelling out to the actor as s/he’s acting.

To add on what “What? Me? Worry?” said, when editing footage, you don’t want any background noise. This is especially true with dialogue. If two characters are having a conversation and a plane flies by and if you need to re-record a part of the footage where an actor messed up, you don’t want to have the sound of the…