Have you seen the "nose blind" commercials that Febreze ran a little while ago? It's like that. We can grow blind to things in our business when we're in it.
Les Brown said it perfectly in the quote above. We need outsiders because we can't objectively perceive ourselves or our businesses. We need outsiders to do that for us.
For this reason alone, there is incredible value in getting honest feedback from an outsider. They can see the whole picture and how pieces can be moved to optimize your business. It's not because they are better, it's simply that we need someone outside of the picture to assess it.
Welcome outsiders' perspective. Their feedback may be the key to your growth.