Why would a defeat in a location like aehgospotami be so damaging to the Athenian economy?

A defeat in a location like Aegospotami was damaging to the Athenian economy because it was a riverside town (both of the same name). Losing control of Aegospotami lost Athens communications with the empire and its seaside access to grain, placing the area under Spartan control. Winning the Battle of Aegospotami allowed Sparata to destroy the Athenian navy.