A sitemap is a file where you can list the web pages of your site to tell Google and other search engines about the organization of your site content. Search engine web crawlers like Googlebot read this file to more intelligently crawl your site.
Also, your sitemap can provide valuable metadata associated with the pages you list in that sitemap: Metadata is information about a webpage, such as when the page was last updated, how often the page is changed, and the importance of the page relative to other URLs in the site.
Most bloggers need a sitemap because:
You will want to have the Yoast SEO plugin for this.
UPDATED 3/18 due to Yoast plugin update.
In the backend of WordPress:
Go to Google Search Console.
Click on Sitemaps.
Click on Add/Test Sitemap.
Add sitemap_index.xml in the box provided.
If test successful, repeat clicking on SUBMIT instead of TEST.