Solar powered fan normally would run all the time when the sunlight is hitting the solar panel but there are times when you may want to keep the warm air in the house such as in cold winter months or on rainy days so it is good to have the on/off so the fan can be turned off.
Another time you may need to turn off the fan is when you are traveling out of town and you do not want to let the fan run while you are away as this will not serve any purpose. Turning the fan off will extend the life of the motor significantly and also save money on gas bills.