As explained, the idea (of some experts) is that you are supposed to look properly each time and identify whether anyone is at any risk from what you're doing, and will therefore benefit from your indication. If you look and there isn't anyone there, you don't need to indicate. You would probably lose points on an advanced driving exam for indicating with no one there as it demonstrates unthinking behaviour.
The counter-argument is that you might look and still miss something and somehow that person would have benefited from your indication. There is a little bit of merit to this.
I don't have much of an opinion either way. People get into big fights about this and I enjoy watching them kicking each other to death about it. Sat in my car in an empty field with my hazards on.