A search engine crawling spider sees your site a lot differently than you do. Having a sitemap on your website helps the spider better index your site and get to all the pages of your site easier and without problems.
Before I start you must first understand how search engines crawl your site so that you can begin to understand why having a sitemap is very important and beneficial to your SEO.
How Do Search Engines “See” Your Site
Search engines and crawling spiders do not see your website as you and I would see it. We can see and understand images and even read text that are inside images. Search engines have trouble with that task. They can read an image but unless you tell them what that image is about they don’t understand it otherwise. You can do this by adding an alt text tag on them thus telling the crawling search engine what the image is about.
Below is a brief video describing and explaining how you can see what the search engine sees and how to better optimise your site so that it can be user friendly but also crawl friendly.