Why Use Robots.txt File ???

"Robots.txt" is a regular text file that through its name, has special meaning to the majority of "honorable" robots on the web. By defining a few rules in this text file, you can instruct robots to not crawl and index certain files, directories within your site, or at all. For example, you may not want Google to crawl the /images directory of your site, as it's both meaningless to you and a waste of your site's bandwidth. "Robots.txt" lets you tell Google just that.
There is a hidden, relentless force that permeates the web and its billions of web pages and files, unbeknownst to the majority of us sentient beings. I'm talking about search engine crawlers and robots here. Every day hundreds of them go out and scour the web, whether it's Google trying to index the entire web, or a spam bot collecting any email address it could find for less than honorable intentions. As site owners, what little control we have over what robots are allowed to do when they visit our sites exist in a magical little file called "robots.txt."
A robots.txt file on a website will function as a request that specified robots ignore specified files or directories in their search. This might be, for example, out of a preference for privacy from search engine results, or the belief that the content of the selected directories might be misleading or irrelevant to the categorization of the site as a whole, or out of a desire that an application only operate on certain data.

The location of robots.txt is very important. It must be in the main directory because otherwise user agents (search engines) will not be able to find it – they do not search the whole site for a file named robots.txt. Instead, they look first in the main directory and if they don't find it there, they simply assume that this site does not have a robots.txt file and therefore they index everything they find along the way. So, if you don't put robots.txt in the right place, do not be surprised that search engines index your whole site.