Search Engine robots and human visitors read websites differently. For example, humans see only the content and images, and search engines see hidden information often placed as scripts or behind elements on a page like in a footer or header. Chances are if you find that a page or website is not optimized properly for search engines, this might be causing lack of visibility or indexing or other problems with ranking for your site.
This is why setting your website's pages up with proven strategic SEO best practices can greatly increase the probability that the pages on your website will rank and be visible so that human visitors will visit and visit often is crucial and that additional SEM will work even better and in many cases lower the costs of campaigns. This holds true with Google Adwords when relevancy of the page correlates to cpc and efficiency of how particular ads are served up. And since most business owners are usually concerned about not spending more money and would rather make more money, it makes sense to make sure that a website's pages are optimal for marketing purposes.