Answer: To announce the time
Though an officer may have had a pocket watch, by far the majority aboard would have no means of telling the time, apart from the ringing of the ship's bell. The bell was struck every half-hour, the number of strikes increasing by one each half-hour up to a maximum of eight. "Eight bells" marked the end of a watch and the beginning of the next watch. The strikes were grouped in twos, giving the classic "ding-ding, ding-ding, ding-ding, ding" pattern at seven bells, for example. All watches were four hours long except for the two two-hour Dog Watches between 1600 and 1800 (First Dog), and 1800 - 2000 (Last Dog).