Robots.txt file is retrieved by search engines, like Google and Bing, and used to state what pages need indexing and what must be ignored.
A robot (also known as Spider or Web Crawler) is a program that automatically traverses the web's hypertext structure by retrieving a document and recursively retrieving all documents that are referenced. Don't forget that instructing search engines bots help you adding visibility to your site and let people reach you in a more efficient way excluding irrelevant contents.
Only if search engines know what to do with your pages, they can give you a good ranking. RapidBot is a must-have SEO tool!