Firstly, let us realize the potential of search engines in today's society, we all know that "Google'" currently powers "Yahoo!'", and we all know that "Overture'" provides it's listings to may search engines. Google, being the prime focus of my current work, pulls in an amazing 150 million different search queries everyday, or at least so they claim. Add that on to Yahoo's many million searches per day and we're talking about a lot of traffic. This is important because these Google results are free to be listed in, meaning that with the right properties, your website has the potential to hold the number 1 position for any given search term regardless of money (ignoring the sponsored links).
With such a great deal of traffic in search engines, everybody wants to be listed in the number 1 slot. This is the nature of competitiveness. This forces webmasters & SEO's (including myself) to analyze Google and other search engines to determine what properties are required in order to rank number 1 on the results pages, this in itself is still not a problem, but when some conclusions are derived from the analysis of the search engines, we are usually left some with a result requiring some page modifications in order to improve the ranking of those pages, and this is where some webmasters & SEO's get confused.
In the attempt to modify their pages to rank number 1, webmasters will often sacrifice how usable a page is to human visitors, and this is the most lethal mistake anybody could make. When a robot visits your site, it will see a whole pile of text, and based on that text, will rank your site for different terms. This text will have no formatting, in the graphical layout sense, although the weight of some text is measured differently.