Perhaps because this was to take place during a war. They were flown out of harm's way,which is how they landed on the island. The plane couldn't function with the type of warfare going on, so it crashed, killing the only adult(s) on board. Perhaps that is the reason he chose a man with a parachute. The reader could believe it to be their pilot or the man could be someone else who was brought down near the same time they were. The man represents humanity and what has happened to it. It all ties into war as well. War dehumanizes us as people. We revert to our savage selves and only care about survival in such situations. This is the journey that Golding puts the boys through. So the decomposing man represents the beast. The beast is really the evil in us all. When we are stripped of our love and compassion for each other, we are all just like a decomposing man on a hill.
The parachutist is a handy convention because he represents the outside world and its war, thus what is wrong with mankind in general. Also, the parachutist, because of the strings attached to the parachute and the parachute itself, can be made to physically resemble some sort of beast or animated being. Golding wrote the book not long after the end of the second world war. He felt that the war revealed the underlying savagery in all mankind. The story begins with the boys fleeing that imperfect world of war. The parachutist reminds the reader that the imperfect world still exists out there beyond the boys' microcosm of society. The boys have been having nightmares, especially the little ones, of beasts. When the parachutist is seen from a distance, the wind catches the parachute and causes the dead man's body to move as though under its own power. This gives credence to the boys' fear that there is a tangible beast on the island. This allows them to stop looking for the beast outside of themselves.