To have an SEO strategy, you need a plan. Sitemap is a word that gets tossed around in the industry like so many other terms and acronyms but can have varied meanings depending upon its use and the person using it.
For SEO, the sitemap is a listing of all pages but with a high focus on the pages you are intentionally trying to rank.
Not all pages are equal.
Not only are not all pages intended to rank, but even some pages are designated to other purposes. Some are meant for navigational purposes, some are intended for attracting quality links for the website while others are meant for strictly ranking.
Starting with a basic sitemap is a first major step for matching up the different breakdowns of products and services and matching them with the desired keywords you want the website to be found for in Google.