(a) Learning in any area means learning, among other things, how much there is to know. This makes one feel humble (at best) - yet in reality one's own knowledge is probably way above average by the time you get to this perspective.
(b) Some people who appear competent (and in some cases think themselves competent) really aren't. Don't assume too much in either direction!
Impostor syndrome can mean that you have difficulty taking pride in your work (since all you can see are the problems with it), which makes it difficult to share your knowledge - which isn't good for other people who might want to know about it and learn from it, and isn't good for your reputation and career growth.
Impostor syndrome can also mean that it's hard for you to ask questions (and admit that you don't know something!), since you're worried that you don't know as much as you should, which makes your work harder.
Not taking pride in your work and not asking questions also means that you're not really being the best role model for people looking up to you - your coworkers, people who work for you, volunteers who help with your project, etc.
(This is based on what I learned from the impostor syndrome presentation described at http://adainitiative.org/2013/06/kicking-impostor-syndrome-i... .)
It's very true, though. Historically men are not allowed to have feelings or ask for help or show weakness. I think this is changing, but I have zero evidence (even anecdotal) to draw from.
I totally agree with "If I don't make things as good as I can, why do them at all?" as I've thought along the same lines.
The worst part are the mistakes that inevitably occur during each 'phase'. When you're riding the ego trip, it's very easy to slip and make a fool of yourself or cause someone else feel stupid when they're not, while on the other end of the spectrum you make poor decisions because you feel like you're not good enough.