If you don't have one, it could generate a 404 error and redirect a search engine to your default 404 error page and confuse it. No harm in having one and it is in fact advantageous to have one.
It can complement your sitemap, there is a code you can put in robots.txt where it helps the search engine locate your sitemap.
Hi, It is just for search engine to prevent any page of directory to crawl. it doesn't have any bad effect.