I believe the original poster is asking about the description under the link in Google results, and why sometimes the Meta Description is used and why sometimes text from the site is used. The poster wasn't asking about the importance of Meta Description to Google.
If I'm not mistaken, Google uses the Meta Description as the description if there is one present in the code. If there is not in the code, it will use text from the actual webpage. I might be wrong about this.
I do know however that Google does not measure rank by Meta Data, but does definately use it for descriptions.
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