There has always been confusion on if Google uses the meta tag data. Some say Google uses the meta description tag, some say they don't. Most say, Google doesn't use the meta keywords tag. In any event, Google's Vanessa Fox said that you should use the meta description tag.
In a Google Groups post, Vanessa responded to a perceived indexing issue as follows:
Looking at your site in the search results, it appears that your pages would be well served by meta description tags. For most queries, the generated snippet is based on where the query terms are found on the page, and in those cases, your results are fine. But for some more generic queries, where a logical snippet isn't found in the text, the generated snippet seems to be coming from the first bits of text from the page -- in this case, boilerplate navigation that is the same for every page.
In summary, by adding a meta description tag, a unique one, for each page, Google will use that information as extra criteria to determine the uniqueness of the page. That is how I understand it. Otherwise, Google will use the top text of your page's content, and that can potentially be your top navigation or worse. This comes in handy for conducting site: command searches with no keyword specific data given after the site command.
Forum discussion at Google Groups.