As you say, Atlantic Specialty Coffee is a coffee importer. They were an outgrowth of Atlantic coffee, based in New York, and that's the reason for the name. Nowadays, both are part of the ECOM group, a multinational coffee (and other agricultural commodities) trading network.
One reason Atlantic Specialty is based in Hayward, instead of back east, is that there is a tradition of specialty coffee importers being based in the bay area. Royal Coffee is based in Emeryville, as was Holland coffee. Now, Volcafe Specialty and Interamerican have offices there too. The San Francisco bay has been the biggest port of entry for specialty coffee since the beginning. Knutsen coffee, thought by many to be the very first of the Specialty Coffee importers (Erna Knutsen invented the phrase "Specialty Coffee") is based in San Francisco.
These companies import and sell coffee to lots of roasters of all kinds across the U.S. Sometimes, they import coffee on their own to sell to roasters out of their warehouse. Other times, they import coffee as a service to roasters for a fee.
The staff at Atlantic are among the nicest and best in the specialty coffee industry. I was just in those very offices a few weeks ago, tasting and talking coffee with those fine folks. If you drop by there, please tell them I say hello
Peter G