The sighting of the beast by Samneric tremendously affects the plot, because it stirs up and intensifies the fears of the boys on the island. In turn, Jack plays on their fears and uses Ralph's inability to stop the beast as means to challenge Ralph's leadership. The boys' foray into the woods after the sighting demonstrates a shift in power from Ralph to Jack, as Jack uses the boys' fear to increase his own importance as hunter and leader in the tribe. Another important development which occurs as a result of the sighting is the discovery of Castle Rock, which later becomes Jack's fortress stronghold by the end of the novel.
The sighting by Samneric of the "beast" on the mountain is a turning point in the action because up until that point, Ralph has been able to maintain that there is no beast on the island--it is only the boys' imaginations. During the expedition to find the beast, Jack is able to show himself as a more fearless leader than Ralph. Although both Ralph and Jack run away from the dead paratrooper when they catch sight of it, when they return to the beach, Ralph admits his fear, but Jack doesn't. Ralph admits they are beaten because they can no longer keep the signal fire going--because the beast is where their fire was. Ralph then insults Jack's hunters by calling them "boys armed with sticks" and implying they are no match for the beast. Jack calls a meeting by blowing he conch and tells the boys that Ralph is a coward. He calls for a vote to depose Ralph, and although the vote fails and Jack leaves on his own, he soon draws almost all the biguns to his tribe.
Simon is the only boy who is undaunted by the beast. After his vision, he goes up to investigate and loosens the lines that have trapped the corpse. This is what causes him to come back to camp after dark just as the boys are doing their wild dance, which results in his murder.
One could imagine a different outcome, in which Samneric have the courage to investigate the dead parachutist from the beginning, Jack does not get the upper hand over Ralph, and Simon is not murdered. So Samneric's sighting of the "beast from air" has a significant impact on the plot of the novel.