If you don't have the redirect, Google will see two URLs, one at http://yoursite.com/ and another at http://yoursite.com/index.php, both with the same content. This is called canonicalization, where a canonical URL causes the same page to be displayed for multiple URLs. This will result in one variation of the page getting filtered out. When that happens, you lose the benefit of all the links pointing to that page.

By using the redirect, you merge the pagerank of the two different URLs into a single page that all spiders and visitors see.

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