What is the advantage of Canon EF-S lenses over EF?

I know EF-S lenses are designed for certain Canon cameras (XT up to 40D), but what is the advantage of them? I was originally made to understand that the EF-S lenses took into account the 1.6x crop factor so that the EF-S 17mm-85mm was indeed effectively a 17mm-85mm equivalent and not effectively 27mm-136mm. The more I read, the more I'm beginning to feel that this is not the case. If this is not the case, then why would I want an EF-S lens as opposed to the more widely compatible EF lenses?