Ashes have some various uses and have high demand for MEMBERS. You can earn more RS gold when you sold it at the Grand Exchange
Ashes from bonfire are useful especially for RS gold sell
Bonfire is a training method to improve the fire making skill. You can add more logs to prolong the fire. This training method can be available for both MEMBERS and FREE players. Other than training, it can be use for Cooking. Once the bonfire runs out, you can collect the ashes for either use it or to sell it to earn RS gold.
To earn some RS gold using ashes, look for a cheapest log you can find at the Grand Exchange (Willow log is the best one you can find). Buy as many as you can or fill up your inventory and storage with logs. Now start making bonfire just by clicking the logs. When the fire burn out on the bonfire, you can now start collecting the ashes. Repeat the process until all the logs has been turned into ashes. Sell the ashes at the Grand Exchange to earn more RS gold.