A helmsman is responsible for steering a ship, no matter if merchant, navy or pirate, each ship will have at least one helmsman. While a secondary helmsman might steer the ship at night, the primary helmsman will take charge during engagements, docking and bad weather.
Helmsmen are rarely the adventuring type, so once promoted they are likely to remain on the same ship until they rise to first mate or captain. This also means that helmsmen are hard to come by, and so if sudden need arises, someone has to step up and meet the challenge.