Humility is great, but when people call something a "syndrome" (a word usually associated with disorders/diseases), they usually mean that the thing is interfering with your life - humility taken too far, to a degree that is making you feel worse and causing problems for you.
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.
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... .)