Because opportunities seldom knock twice
When you look at the stunning websites of great companies like Apple, American Express or GE, you probably don't think to yourself “Geez, why did they go to all that bother?” In many ways its obvious.
A better website leads to better understanding which leads to people thinking - “now, that's a great company. I like what they are doing” or “oh, that's so cool, I would love to have one of those”, or “hmm, this is interesting, I think these are the guys for the job.”
A better website will easily convince people that you're a professional company that can solve their needs.
People use websites for many reasons and each reason is a golden opportunity to convince, so don't waste it!