Why we use robot.txt file in my website

robot.txt file:

A robot.txt file is represents a robots file that allows or disallow your website's content that you don't want to be indexed by Google search engine robots (also known as "bots"). By using robot.txt file on your website you can recover or improve your web pages by Google robot.txt. In your website, Google robots are indexed your all website contents, images or other files, you can allow or disallow Google robots according your robot.txt file. Example of your website robot.txt file:  
http://www.website_name.com/robots.txt

Note:

In your websites with multiple sub-domains, you have to add robots.txt file in each sub-domain with own robots.txt file. 

The following website url can helps you for create a robot.txt file:

http://www.seochat.com/seo-tools/robots-generator/
http://www.mcanerin.com/EN/search-engine/robots-txt.asp
http://www.submitshop.com/seo-tools/robots-txt-generator
http://multitoolbox.com/webmaster/robotstxt-generator/ 
http://tools.seobook.com/robots-txt/generator/