Meta descriptions are HTML attributes that concisely explain the contents of webpages on Google's SERP's. So, Google’s algorithm must take these meta descriptions into account when determining search rankings, right? Well, not so much.
In fact, Google announced in 2009 that meta descriptions and meta keywords have no bearing on search ranking and they haven't indicated otherwise. But, that doesn't mean your meta data isn't vital to your website's success.
Meta descriptions are valuable because they do present a major opportunity to separate yourself from competitors and convince searchers that your page is worth reading. Getting searchers to click on your page instead of clicking elsewhere will increase your clickthrough rate which is a major factor for Bing and other search engines. Honestly, Google has never given a definitive answer to questions about the relationship between click through rates and page rank. But, they conduct live experiments to identify changes in users click patterns on competing pages.
One can safely assume that all search engines operate according to the same fundamental concepts.