The role of a film poster is to demonstrate the concept of the film to the audience, from a rough overview of the film without giving the key aspects of the story away.
In many circumstances writers are not present during the shooting stages, which can lead to their vision being lost. Having a concept or even a series of concept posters helps the director and assistant director maintain the essence of the film.
Even for writers who later direct their film, concept posters assist in ensuring continuity especially as more inspiration strikes for another film idea.
Concept posters have the additional role of garnering interest in the project from investors and testing the market for the film idea itself alongside bringing in additional talent.