To the lighthouse...

What is a lighthouse? In a seaside town, the lighthouse is a collective effort: the inhabitants of the town pay for it, but once it's paid for, the upkeep is minimal (some have a bearded man in a sweater at the top, others are cheaper and automatic). The lighthouse acts as a guide for lost seafarers, and the point is to keep ships from dashing themselves on the rocks. But lighthouses can't charge the people who actually USE them, or to put it differently, who uses the lighthouse? What exchange or gift occurs when the lighthouse "disseminates" its signal? Interestingly, lighthouses seem to have two lives, a night life (its intended time of activity) and a day life (a time when tourists on the land investigate and photograph it).