Before there were those showing off skill capes, firemaking was mainly used for quests. The idea when your a low level though is to fish then cut a tree then cook the food. Repeat for awhile and you can get your fishing and cooking and Firemaking level up.
People who get it up to 99 are just getting it for the capes as simon said but meh. Its a nice cape, iv tried to go for it myself but it was too time consuming and i sold the logs. Now the logs have quadrupled in price making it little harder to achieve.