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.
If a single rider were to hold the lead consistently, they would eventually become fatigued and the group's speed would drop. Conversely, distributing the lead among riders ensures that the workload is evenly spread out, potentially leading to faster overall speeds or reduced individual effort.