Ships use this alphabet to communicate with other ships at sea. In a shripwreck situation, these flags could be used by the castaways to communicate with would-be rescue vessels. For instance, if there were submerged rocks between the beach and the rescue vessel, the castaways could use these flags to safely guide the ship to shore.
In the age before radio communicationss, a system of flags such as this was used frequently so that ships could communicate with each other. Knowing this alphabet was a mandatory skill for any sailor, and for the contestants of Survivor-Galveston, it will be necessary to know this alphabet to complete one of the tasks.