Search engine optimization helps to increase site traffic by getting higher search engine rankings. The META description tag has been shown of value in white hat SEO techniques. It is essential you have different page specific META description tags for each page on your site (Do not use the same description for all posts).
WordPress by default has no META description tags for individual posts. I tried the Head Meta Description wordpress plugin that provides an automatic meta description tag for your blog, inserting a dynamic description depending on the query-type. I easily configured it to display the first 20 words excerpt of the article as the description. It worked effectively and is very useful to automatically add a dynamic description to each page.
Problem: It is agreed that the META Description tag should be representative of the page it describes, after all that is the whole idea. But when you decide to extract the excerpt of the first 20 words, the resulting description may be nothing related to the page concerned. A mismatch of the post content and the META tag may actually reduce the sense it make to search engines and reduce the search rankings.
Solution: So if you decide to use these plugins, it is a good idea to change your writing style such that the target keywords are at the start of the article. Include the main content in those 20 words so that the description makes sense.
You can edit the number of words to show in the description from 20 to any number you like via the plugin editor. I tried 40 words too, but many of my older posts did not have the targeted content in the first few words and made no sense (and I did not have the courage to edit thousands of posts to fix this). Now I am consciously trying to get the content synopsis in the post excerpt.
Here is a large collection of wordpress plugins for META tags that you can use to automatically generate SEO targeted Meta tags.