A robots.txt file is a plain text file that provides directives to web-crawling robots on how they should crawl a site. This file can be used to prevent the indexing and public display of certain parts of your website.
Robots.txt files are not strictly mandatory. Without one, a spider will simply crawl the site based on its default guidance. However, most websites benefit from including a robots.txt file to help efficiently guide and manage crawlers.
A robots.txt file can be used to encourage or discourage the crawling of specific pages. For example, if a website maintains a private staging or development area that should be blocked from indexing, you would use this file to prevent that page from ever appearing in search results. Robots.txt files can also be used to ensure that certain important pages are prioritized for crawling, or to hide poorly optimized pages from being ranked temporarily.