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Many reasons. You don't want the admin pages of your site indexed, for example. You may not want all of the search queries that people perform on your site search to be indexed. You don't want or need your cart checkout being indexed for an ecommerce site. You don't want a print version and a web version of the same document indexed, so you exclude the print version from being indexed. Your site is in development, and you don't want it being indexed before it is ready. For robots.txt in particular, some search engines now respect wildcards and you can exclude some session IDs via robots.txt. OSCommerce is real bad about creating session IDs and getting those indexed, then you have tons of different URLs indexed for the same page. http://www.cogentos.com/bloggers-guide-to-using-robotstxt-and-robots-meta-tags-to-optimise-indexing/ is a post that explains some of the reasons to use robots and no-index on a Wordpress site.