A relevant keyword domain by itself is pretty much useless for SEO. Search engines algorithms pay much more attention to factors like inbound links and page content than the domain name keywords. But having that perfect premium domain can be the ticket to better rankings.
How so? You can use the keywords as your site name. Say for instance I sell online Passport photos and buy PassportPhotos.com. I could name my site “Passport Photos” and legitimately ask that my links contain those keywords.
Granted, a lot of my links would be titled PassportPhotos.com, but I would still pick up some of the former. A keyword domain is the cheapest way to get relevant links in the long run.