Well, Ralph and Piggy reiterate the main reason for the fire's importance throughout the novel. Here's Ralph speaking in Chapter 2 about the importance of keeping a signal fire going:
“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.”
“We’ve got to have special people for looking after the fire. Any day there may be a ship out there“–he waved his arm at the taut wire of the horizon–“and if we have a signal going they’ll come and take us off.
In fact, later in the novel, Ralph even goes so far as to say
The fire is the most important thing on the island.
The fire plays a secondary role: it provides hot food, and it allows Jack to cook the pig meat he provides. Of course, part of the importance of the fire to the novel is about the tension between the two roles: its role with Jack as a food-cooker and an atmosphere-builder (as well as, later as a violent threat) - and its role with Ralph as a signal.
As the novel continues, the boys shift from Ralph to Jack, and from signal fires towards pig hunts. Ralph has to try and restate his case:
“Look at us! How many are we? And yet we can’t keep a fire going to make smoke. Don’t you understand? Can’t you see we ought to—ought to die before we let the fire out?”
The first sign that the boys aren't going to control the island is when they create a big fire they cannot control. One of the earliest tension points between Jack and Ralph is when Jack lets the signal fire go out. And, at the end, the fire - and this is its other key purpose, as a symbol - swallows the whole island. What does the fire symbolise? Hell, the devil, the darkness of man. The fire, in the end, turns the paradisical island into a figurative burning hell.