What is the function of a wave-swept rock?

 title: 'A lighthouse with a tall tower on the coast'

A wave-swept rock's purpose is to have builders mark some terrible danger-spots[1]. These spots can be on the mainland or isolated sea-rocks[1].

They are difficult for builders to overcome because the builders are at the mercy of the weather[1]. The rocks may be accessible for only an hour or two per day in calm weather[1].