I copied the following text from Dana.com
“Power assisted steering is present on most vehicles. What is not commonly known, however, is the fact that some power steering systems reach temperatures that significantly reduces the life of the system.
The power steering system creates pressure, in turn creating heat. The rise in the temperature of the system can create loss of assist as well as leakage. This primarily occurs when the temperature of the fluid exceeds the specification of hoses and of the seals in the pump.
Power steering oil coolers are commonly found on higher performance or specialty vehicles, vehicles with increased trailer towing capacity, large car platforms, SUV’s and Trucks.
Automakers continue to stretch warranty periods to win the end consumer’s business and power steering oil coolers are one of the components used to achieve these extended warranty periods and ensure a robust product.”
New power steering systems employ electric pumps which pretty much eliminate parasitic losses on continuously operated pumps driven by the engine. The hydraulic pressure is maintain by operating electric pump only as needed. I have electrically driven power steering pump on my TDI VW Jetta. I believe that primary purpose for using electric power steering system is reducing fuel consumption.