I agree with Felgall that the above statement is the core objective. But, there is a difference between telling them that a pretty website will make money, and actually showing them that in a factual, evidence-based manner.
There was a period of about 2 years where my company was having some pretty major growing pains and we decided not to pursue any new business at all. Instead, we struggled just to handle the increasing revenue and workload from our core group of around 12 clients. This was around 2009 and at that time, there were tons of people talking about web standards, css-only sites, etc.
Our website, which had been a content-rich Joomla site, was turned off and a somewhat sloppy hold page was put in it’s place.
Over that period, I think I got about 100 solicitations from web designers who felt the need to explain to me why our website was losing us money. They explained why it was an SEO disaster, and why the lack of code that will validate was hurting us in the end. They explained why we were losing sales, etc. It went on and on.
I thought the same thing every time: the person is telling me what they want to believe is true about my business and my website, not what is actually true. The reality was that we were trying to get the business streamlined and tidy so that we could sell it, which we did. Most assets of the company were sold but the name, domain, and website remained ours to repurpose in the future. Thus, it was a good idea to put a hold page on the site.
There are plenty of ‘ugly’ pages that sell very well. And plenty of pages that don’t validate that work just fine. There are plenty of pages with no-so-great navigation but it doesn’t matter in their particular circumstances.
The bottom line is, if you are going to tell a business how a website is going to make them more money then you’d better have a very clear idea of how their business works, what their priorities are, and what they want out of their website.