I received this bit of advice about the meta description tag recently:

Meta descriptions are used by Google probably 80% of the time for the snippet. They don’t help with rankings but you should probably use them. You could just auto generate them from the first part of the question.

The description tag exists in the header, like so:

<meta name="Description" content="A brief summary of the content on the page.">

I'm not sure why we would need this field, as Google seems perfectly capable of showing the relevant search terms in context in the search result pages, like so (I searched for c# list performance):

In other words, where would a meta description summary improve these results? We want the page to show context around the actual search hits, not a random summary we inserted!

Google Webmaster Central has this advice:

For some sites, like news media sources, generating an accurate and unique description for each page is easy: since each article is hand-written, it takes minimal effort to also add a one-sentence description. For larger database-driven sites, like product aggregators, hand-written descriptions are more difficult. In the latter case, though, programmatic generation of the descriptions can be appropriate and is encouraged -- just make sure that your descriptions are not "spammy." Good descriptions are human-readable and diverse, as we talked about in the first point above. The page-specific data we mentioned in the second point is a good candidate for programmatic generation.

I'm struggling to think of any scenario when I would want the Google-generated summary, that is, actual context from the page for the search terms, to be replaced by a hard-coded meta description summary of the question itself.