Sitemap helps SEs know about any updates that you make on your site. It is not guaranteed that the search engine bot will immediately index the updated pages, but, the changes will surely be indexed much faster compared to having no created sitemap.
HTML sitemap helps to visitors to understand the navigation of the website & XML sitemap helps to know about the updated pages of the websites & helps to get indexed pages by the crawler
There are two types of sitemap which we usually created 1) HTML Sitemap 2) XML Sitemap HTML Sitemap is generally created for visitors, to understand the site navigation. Its a simple text XML Sitemap is generally created for google crawler to know easily how frequently our webpages are updated on a website. We can update our XNL sitemap every month so that crawler can find new content and the changes we have made it, to help for indexing.