YES. A website may benefit from a site map. Even though pages can be indexed from a website following quality back links, it cannot be confirmed that all the pages will be indexed, due to variety of reasons. But if you have a site map ready, with all the information in place, one can confirm that all the pages are indexed on Google and the other search engines.
There have also been instances where sites without sitemaps have also made it to the search engines without much difficulty. That’s fine, but if you have a website that is not well reputed yet on the search engines, and have fewer backlinks, then submitting a sitemap may help. And if you have a website with large amount of content, a site map gives you an opportunity to organize the content and get the best ones out there on the search engines.
BUT, site maps are not required. Site maps won’t guarantee all your pages are indexed. Creating a Site Map will not fix poor coding, website navigational, keyword research, and Meta Data. In fact, if you need a site map to index all your web pages it means your web developer & programmers did a poor job building your site. It also means that you are most likely not benefiting fully from internally links.