A Web site can make you look more professional or experienced than you are. However, sometimes it can make you look like an idiot even if you have a good product or service. This is especially important for small business who don't have big marketing budgets and rely on the Web for part of their lead generation.
The site below is a pizza place that I've never tried that's a few block from our house. I decided not to order from them simply because their Web Site sucks. It's a good thing they've littered their site with Google AdSense ads (and a LOT of other things) because I can't imagine a lot of people will buy their pizza based on the site itself.