tl;dr Give a useful description of the page.
This is the snippet of information that search engines often show under the page title in SERPs, so you want to make your potential visitors know that your's is the page they are looking for using this tag.
I don't believe it has much if any effect on ranking, but it is certainly important for "Click through rate", and remember, you are trying to appeal to human beings here, not robots.
Bing recommends a description length of 25 to 160 characters. That's because if it's too long it may be cut short. Too short and the SE may ignore it and pull up some other content to show in the SERP.
Google recommends that the description (and title) for each page on a site is unique.