Each vehicle has fluid and most have serviceable filters that require changing/servicing at either 30,000 or 50,000-mile intervals. As fluids age, they break down and stop lubricating as needed, so this service helps prevent any expensive breakdowns of the internal transmission components.
Differential Fluid Services. The rear differential, as well as the front (on 4wd and AWD vehicles), require scheduled fluid services to replace aged fluids to assure proper lubrication of all internal gearing, normally every 30,000 miles on most vehicles.
4wd and AWD vehicles have transfer cases that require scheduled fluid services to replace aged fluids to assure proper lubrication of all internal gearing as well, normally every 30,000 miles.
Vehicles are either manual, power or electric steering. If the vehicle is power steering, it is fluid based and also requires scheduled fluid flushes to replace aged/worn down fluid with new in order to prevent damages to internal components.