Things like the DESCRIPTION and KEYWORDS fields basically help some search engines (e.g. Inktomi but not Google) categorise your site, and can enhance your search engine results.
If you rely on Google for most of your traffic, these fields probably won't make much difference, but it's good discipline to do them for the other SEs because you never know when someone with a large pile of cash and a preference for obscure search engines might pop by
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