The sitemap generator creates and submits a sitemap to Google (and other search engines) so that all of your website's content is searchable. Without a sitemap generator, you have to rely on a search engine's crawlers to come by and index your site, which may be weeks or even months away. Any updates to your site's content made after the most recent indexing will not be available until the next indexing. A sitemap generator speeds this process up and makes your website's content available much sooner.
Learn more about the sitemap tool here.