If one rider were to stay at the front all the time, they would tire and the whole group would slow down. If the lead is rotated, the effort is distributed across the group and the speed can be higher or the individual effort less.
By keeping the lead position for an extended period, one rider might tire, slowing down the entire group. Alternatively, rotating the lead allows each rider to contribute their share of effort, which can either increase the group's speed or decrease the intensity of individual efforts.