The practice of using the hyphen - (dash) is a throw back to the old days when the Yahoo directory was king. In the Yahoo directory they still use the words in the title, description and URL to list your site for searches. If your URL had 2 words in it, your site showed up for searches for those 2 words together without a space. but not for them individually unless Yahoo had them in your title or description.

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http://www.photostamp.org/
The End