Each fluid used in the vehicle serves an important role:
Fluids such as engine oil and gear lube work to lubricate the moving parts of the vehicle.
Engine antifreeze transfers heat from the engine to the vehicle cabin in cold weather, or to the outside air in hot weather.
Brake fluid transfers the pressure of the drivers foot on the brake to the brake components at each wheel, and automatic transmission fluid and power steering fluid are used for lubrication and to direct hydraulic pressure to engage gears or help with turning the steering wheel.
Each fluid has certain properties that can deteriorate as the fluid breaks down.
Heat is a large factor in the breakdown of these fluids. As a result, fluids need to be checked or replaced periodically in order to maintain a properly running vehicle.
Generally, in a fluid replacement service fluid is run through the whole system, flushing old fluid and replacing it with new fluid.