Site map is just like a road map which tells your search engine (google, bing and others) the pages you deem fit to appear when users make a search. Posts included on the sitemap are indexed and ranked on google to appear as search results.
Some websites do not use it and i do not really think it has an adverse effect but i have used it on all of my websites.
if you are a wordpress user, you can use yoast, google sitemap or jetpack to lay out your sitemap.