Sitemap is important when your site is complex. It allows your audience to understand the structure of your site. Moreover, you should submit your sitemap to Google Webmaster so Google can understand your site.

Go to Admin > configuration > sitemap.

This module provides a site map that gives visitors an overview of your site. It can also display the RSS feeds for all blogs and categories. Play around with all settings.

Master Drupal in 7 hours - Configure SEO - Sitemap

The sitemap is available at http://yourdomain.com/sitemap. You can use this link to submit sitemap to Google Webmaster.