When you construct a website’s layout with tables, it’s difficult to let search engines know which content is important and should be read first. It also pushes important content further down the page while littering your code with unnecessary markup (altering your page’s code-to-content ratio in the process…but that’s another post.)
For example, most designers will create a table with a header cell, then 2 column cells below it for the navigation (left) and content (right) plus a final footer cell for the copyright information (not to mention a dozen empty cells that function as gutters and placeholders). Once they’ve constructed this already un-optimized structure, they proceed to add even more nested tables, making it even more difficult for search engines to determine what is important.
With this layout, the search engines first reads your header (which is probably just a graphic), then your navigation (which probably has deficient keywords like, “About Us, Products, and Contact Us, etc.), and then finally finds your keyword-rich content. The search engine is forced to wade through all of the unimportant bits before it actually finds something valuable to index.
You are better off constructing the main architecture of the site with positionable DIVs. With CSS and DIV’s, you can construct your page so that all of the important content gets read first in your source code, but is still presented on-screen in whatever manner you wish the human viewer to see it.