Brake and clutch hydraulic fluid is hygroscopic (attracts & retains moisture from the air). Absorption rate is approx 1.5 – 2.0 % per year (whether the vehicle is used or not).  6% will adversely affect and cause harm the parts which hold and transfer the fluid.  This fluid must be flushed out and replaced every 2-3 years or damage will occur to those parts and cause many hundreds of dollars to replace or repair. 

Engine coolant (nickname anti freeze) is a special fluid with additives to transfer heat from the engine to a heat exchanger like a radiator or heater core. This coolant with its additives will protect the different metal parts within the engine from freeze, overheat and corrosion, as well as rubber seals (in water pump) and hoses.  As coolant ages during heating and cooling cycles it will develop an electrical charge (which can be measured) also acidic which can also be measured on (Ph) scale. Specific gravity can also be measured to determine coolant condition. Engine coolant must be replaced or damage will occur to coolant pump, hoses, head gaskets, radiator and heater cores. 

Hydraulic oils like power steering, transmission, transfer case, drive axle differential have specialized fluids with additives for each application and design.  All fluids need replacement at certain intervals to allow the systems involved to operate normally and reduce operation problems or failures.