Descriptive phrases are important for search engines. Some have noted this importance in SEO tutorials and books about keyword significance as regards density, prominence and effectivity for engine rankings. Some have proclaimed <meta> attributes for search engines are insignificant. They don’t use keywords. They don’t use descriptions. And, whereas, there is an inherent secondary value for descriptions increasing rank prominence, there exists one value which is ignored when meta:descriptions are not used.
Visitors.
Those who have done search engine queries and found a page from your site are likely to read what follows the page title even if they know your site. Site usability should extend to those who have found your site on search engines. Shouldn’t it.
8 steps to serving better (X)HTML [July 27, 2006] by Tantek Çelik states it correctly,
3. Dump <meta> keywords. Don’t waste any time or space on these, per the Principles of visibility and human friendliness (especially the fact that search engines don’t care).
[And, read his cited article for a simplified introduction of microformat tagging (which shall have an effect with search engines in future algorithms.]
Meta:keywords are deprecated.
However.
He doesn’t use meta:descriptions. Google site:tantek.com
and one can see what Google returns instead of a description. Some of the search engine summaries are descriptive; some are not. He doesn’t need descriptions but he should have them. Not Everyone knows him: someone unfamiliar with his work would be able to immediately identify pertinent information whilst researching a subject for which educating passages may be found on his site.
A List Apart's articles use meta:descriptions. Sometimes. The Elementary Group Standards does not have the recognition nor readership nor authority ranking of those sites; it uses descriptions. Three reasons are involved.
- Usability
- Perceived Value
- Search Optimization
The following examples — Why XHTML™? and The Woody Words — were taken from a simple search of all Elementary Standards pages indexed and cached by each search engine. The examples are from the current popular engines.
Search engines use different character limits in their “page summary description section”; the limits are different per each engine based on algorithms which use a combination of complete words and individual characters in a sentence. They select different words dependent on search phrases used when specific search phrases are used by users. [Note: All search engine once displayed <title> and <description> only in search engine results pages. The following illustrate what they use these days. That will change.]
These two.
- Google presents the meta:description. The maximum number of characters used from the description is 130. Any word that exceeds that maximum number is ignored, so the actual character representation of a particular description may be 127 or 120. The example below presents the complete meta:description.

- However, when the description is less than 130 characters or not used, it will present the description and select secondary words to fit the allotted character limits. The example below presents the complete meta:description, “A small article on woody words.”; a header
<h3>element, “The Woody Words”; and, a segment of the first sentence. 
- The example below illustrates actual Google search results. The phrase “woody words philologia” was used. The meta:description has been presented; and, the first instance of “Philologia” (which in this case has been taken from the navigation).

- Yahoo!
- Yahoo presents the meta:description. The maximum number of characters is approximately 210. The complete meta:description is presented.

- And, approximately 100 characters are taken from sentences in the content should the meta:description be small. The meta:description has been presented but notice the difference between Yahoo! and Google in their selection of secondary text. Yahoo! has taken the second sentence first which is followed by a segment from the first sentence.

- Further, they will include an approximately 100 character sentence with the search term taken from first instance of its use anywhere on the page. The example below illustrates actual Yahoo! search results. The phrase “woody words philologia” was used. The meta:description was presented. And, Yahoo! search results present each instance of “woody words” found in
<p>elements and ignore the<h3>element. 
Then these.
- MSN
- MSN presents the meta:description in an abbreviated fashion, then selects a header element when used and ends with the first text in the content. Small descriptions are used entirely and their character limit is filled with the first—Approximately—100 characters. MSN results are similar to Google's, i.e., meta:description,
<h4>element and first sentence. Notice that MSN has selected a different header element, i.e.,<h4>, since the<h3>text is the page title and they loathe redundancy. 

- Alexa
- Alexa search results are powered by MS Live.


And, then this.
- Ask
- Ask ignores the meta:description but selects segmentation of the first text in the source code which is relevant to the search terms. [Note: “Why XTHML” and “The Woody Words” were not indexed by Ask; hence, these two examples.] And, since the search was for all pages found on The Elementary Group Standards site, these are what occurred. Odd, aren’t they.


So.
Search engines follow vertically all text content as written in the source code. Search engines do not follow style sheets presentation, e.g., { position: absolute } and { margin-left: -9999px } are ignored. Search engines have different algorithms for displaying descriptive summaries. Therefore, text content which comes first should be user-friendly if you choose not to use meta:descriptions.
You can perform your own research on what search engines present for your site. Simply use “site:Insert Your URL” for Google, Yahoo, MSN and Alexa sans quotes; Ask requires “URL site:URL” sans quotes. [Note: This should work on lesser search engines but if it doesn’t you can use their “Advanced Search” form.] You can see the quantity of pages a search engine has cached and/or indexed for your site as well as see what possible first time visitors see. Finally, if one uses clever page titles, meta:descriptions clarify page content. If not, your page summary description on search engines will be disjointed or Gobbledygook. Or, worse.
Everyone should use meta:descriptions.

