The significance is that the description clearly illustrates to the reader exactly how the boys could have been fooled into thinking that the body of a dead parachutist was an evil, living monster.
The words "life-like movement" are used to describe the body when the wind causes some of the creepers to pull on the shrouds. The movement is so lifelike that it even fools the flies that have been hovering around its face. "The tangle of lines showed him the mechanic of this parody." The description also explains how the body has remained in tact for all of this time. The boys are on a tropical island. The heat and humidity should have increased the rate of decay on the body, but the rubber and canvas of the parachute have helped to hold the body together.
It's a significant moment in the story, because it confirms Simon's suspicions that the beast is not an actual concrete thing, but the capacity for evil contained within each boy. Simon decides it's critical to tell the rest of the group what he has found, but unfortunately the frenzied boys think Simon is a beast from the jungle and kill him in their excitement. That act cements the very real presence of the Lord of the Flies within the remaining survivors.