In my case, the answer is easy: when I started as a freelancer, I discovered that I had better reaction when I was wearing geek shirts, than when I was wearing a suit. Somehow, when I was wearing the suit, people thought of me more like a smoke-seller than some other thing, but when I was wearing a geek shirt, then they took me seriously and capable.

So, I believe that it is a matter of expectations. Customers expect from good developers to wear geek shirts, and from smoke sellers to be suit up.