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When a searcher uses a keyword that you've included in your meta description, Google will show your meta description as the snippet for your site in its search results. When you take the time and effort to write them well, you can encourage searchers to come visit your site by letting them know that they will find what they need.
When you don't take the time to write your meta descriptions properly, you can end up with a search result that looks like this:
Now, maybe Google are planning a new feature called Google Ouija to enable people to find personal injury solicitors from beyond the grave, but whoever wrote that descriptions shouldn't make that assumption. It's unlikely that somebody who's lost a loved one in an accident is going to click that listing as they're likely to consider it glib and unsensitive. Naming the page "fetalaccidents.asp" won't help either.
What we should take from this example is that it is vital that your meta descriptions not only contain your keywords, but that they give searchers a reason to visit your website.