The use of sitemap.xml is one of the alternatives to improve the positioning of the site in search engines. The main function of Sitemap.xml is to help indexing pages and categories of a website.
If the site has a large volume of pages, categories, subcategories, products and byproducts, sitemap.xml will certainly be a great facilitator for the search robot (Google) to find each of these pages.
It's important to remember that Sitemap.xml will never solve everything yourself. It should always be accompanied by a good internal link architecture, making the search robot (Google) find and index more pages, and of course, accelerate the process of indexing new pages if your site is modified frequently. You can create a Sitemap.xml through a Sitemap.xml generator (for beginners). I recommend the CIA WebSites Sitemaps Generator .
To improve the SEO of your site, follow these tips:
Create an XML Sitemap
Enclose your XML Sitemap on Google
To subscribe to your XML sitemap on Google, you can follow this tutorial of ecommercebrasil