If search engines find, or follow links to, http://blahblahblah.com and http://www.blahblahblah.com, they don`t actually see them as the same site .... so with the redirect, everything is pointed to the url you choose, and you don`t get penalized for duplicate content, plus inbound links carry more weight.
htaccess is used for a huge list of things that can be applied to your site, besides just passwords. url rewriting is a big one, which not only makes nicer looking urls, but search engines actually prefer that you use them.
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