I like WordPress very much, but the native vanilla installation does have a few limitations that this plugin overcomes. The idea was to have an interface where the end user could edit their website content without knowing HTML, and that’s terrific, but some of us still need access to a number of the HTML elements on the page, most critically: the title and description. This plugin creates a location to enter and edit that data for each page and post.
Search engines also make use of sitemaps. These files are essentially just a list of every page you wish the search engine robot to crawl for information. Now, there are many scripts that exist which can crawl through every single link within a site in order to generate one of these, and lacking that, you could always produce your own manually, but this is extremely time consuming and tedious. Also, the script version will gather a lot of links which you don’t care to have indexed, such as user names, page numbers, and various media. Yoast gives me an interface where I can choose what to include and what not to include in the sitemap, per page, per post, per author, and whatever else. You pretty much need something familiar with how WordPress is put together in order to make a sitemap of any value. As this plugin also provides fields for other page content that the search engines need to index your site properly, it’s an absolutely essential tool to have once your site is past the testing stage.