The most important reason to why you want use the meta description tag is to take control over how the link to your page is presented on search engine result pages (SERPs).
This is how the page about the Easy WP Meta Description is presented on Google’s search page.
The first line is the page title, that is the text between <title> and </title> in the html-code.
The second line is the URL. The text snippet under the URL is what we are interested in now.
If you don’t add a meta description at all Google will try to summarize the page somehow. Often they just take the first two lines which can be an ad or anything.
What you want is probably some descriptive and selling text that make the searchers want to click on your link. It will give you a better search engine click through rate and more traffic.