[I]t’s worth noting that while accurate meta descriptions can improve clickthrough, they won’t affect your ranking within search results.
Even though we sometimes use the description meta tag for the snippets we show, we still don’t use the description meta tag in our ranking.
Optimized Meta Descriptions ARE Important For SEO
The first metric I think Google makes broad use of is direct Click-Through Rate (CTR) from the SERPs themselves. Whether or not a result gets clicked on is one of Google’s and Bing’s first clues about whether any given result is a good match to a query. We know Google and Bing both have this data, because they directly report it to us.