The original purpose of DP was to eliminate helpers in mountainous territory. On the UP the original implementation was in the Blue Mountains. They were limited by drawbar forces to 50 car trains. That limited the amount of tonnage that could be put over the Blue Mountains. By implementing DP they were able to double the capacity of the line without increasing the number of crews.
Later DP was applied to bulk trains. The last trains to be DP'd were manifest and intermodal trains. The longest train operated by the UP was 18,000 ft long test run and ran from Dallas to Los Angeles. It had 4 sets of 3 engines, spaced 6000 ft apart.