The answer to this question can be found about halfway through chapter 2. The boys are meeting together to discuss their situation. The conch is being used, and readers get introduced to the collective fear of the beast, which will continue to grow. Eventually, the arguing about whether or not a beast actually exists switches to the boys wanting two things:
We want to have fun. And we want to be rescued.
The idea of rescue is an important one, and Ralph says that their best chance of rescue is for a passing ship to somehow see that the island has people on it. Ralph states that the best way for this to happen is to have a lot of smoke, and in order to have smoke, the boys need fire.
"There's another thing. We can help them to find us. If a ship comes near the island they may not notice us. So we must make smoke on top of the mountain. We must make a fire."
"A fire! Make a fire!"
At once half the boys were on their feet. Jack clamored among them, the conch forgotten.
The logical reason for a fire is that it helps the boys get rescued. I also believe that the fire idea catches a lot of initial support because they are adolescent boys, and burning stuff is fun.