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While your robots.txt file is not the best means to control search engines, it does have a purpose. To respond to your questions: the file does not "attract" any robots, but robots who do visit can learn a bit about your site and understand what content you don't wish to be crawled you can block parts of your site that you feel have no value for indexing such as Keri mentioned your "print" version of pages, or overlays pages, or login pages, etc. The idea is that you own the website, and you can have a measure of control over it. You can disallow specific crawlers, etc. although it's up to each crawler whether they actually respect your wishes. More details can be read at: http://www.robotstxt.org/

Dr. Pete also wrote a good post about this at http://www.seomoz.org/blog/how-many-links-is-too-many.

If you have 301 redirects of every page in place as the new design is uploaded then the traffic that comes in through the old URLs will be moved by your server to the new URLs.  Also, search engines will follow links to the old URLs and give the new URLs most of the linkvalue. If you don't use 301s on a page then any person who clicks a link to your site on another domain or in a search engine will not see the new page.  They could see the old page if you did not remove it or an error if you removed it.