because if it's changes, Google need follow it, and follow it quick so, they have to check it everyday to make sure they do not miss it.
According to google robots.txt is very important file. so it update the information available in this robots .txt file
A robots.txt file restricts access to your site by search engine robots that crawl the web. for more click here
I have a site logicspice.com and some day ago goolge don't crawl my site home page. google remove my site from Google SEPRs . Then I check in webmaster google show robots.txt unreachable. Please help me to get back in google