They're one of two companies that has either optimized their website to appear in the top rankings of Google, or bought Google ads, so that they appear when you do a search for a magician. Try searching Google for "New York Magician" or "Los Angeles Magician" and see what pops up. They're at the top for those regions, and as soon as you get out of the major metro areas, their results are always at the top, too. Try "Rhode Island Magician" to see a different area.
Since they're first and/or prominent in the Google search results in the US, they'll get a lot of phone calls, so if you're listed with them, you can pick up shows (probably mostly birthday parties) for your area.
I've heard varying stories about how good they are to work with.