When buying a used car it's usually a good idea to change all fluids, just so you know exactly when they were changed, and can start your own fresh service history. This may be part of the reason why the dealer recommended these services.
Like almost any other fluid used in a car, time, environment, and use will eventually cause degradation and decreased performance. Over time, the chemicals in the fluid will break down, usually not to point that it cannot do it's job, but to the point that it becomes less effective.
Power steering fluid does not typically see excessive heat depending on the reservoirs/fluid lines location in the engine bay, but it serves both to actuate the hydraulic steering, and cool the power steering pump. Over time, the fluid can degrade, and although it is a closed system, any exposure to heat or moisture will eventually diminish its performance. Either way, the PS fluid is the cheapest and easiest part of the PS system to replace, so it's a good idea to change the fluid as preventative maintenance. When the PS fluid becomes dark/discolored (as pictured below), it's time to change it.
The rear differential is a much simpler 'oil bath' type of system, but the spinning gears/driveshafts create considerable amounts of heat, and over time cause the oil to break down. It's difficult to tell when the diff fluid needs to be changed, as it becomes discolored almost immediately after the vehicle gets up to speed. If you hear any abnormal whining or whirring noises from the diff, it definitely needs changing. If you put a magnet in the fluid and find metal flakes it probably needs changing, otherwise I would replace it when you first buy the car, then stick to factory prescribed service intervals.
