More and more manufacturers are moving away from hydraulic power steering systems in favour of more efficient electrical systems, but there are still many vehicles available for sale that use the older style of power steering system.
And leaving these cars unchecked for power steering fluid that's free of particles and grit that build up over time will ultimately result in damage to the steering system in a cascading series of events. First of all, the particles can damage rack and pinion seals, and if the fluid leaks out of the reservoir, the power steering pump will run without lubricant and eventually fail.
Making light work of cornering in a car, the power steering system is usually filled with a mineral oil, a silicon-based fluid or a synthetic oil like automatic transmission fluid. At the very least, the reservoir that contains the power steering fluid should be checked at regular intervals and topped up if the fluid level is low.
In the event that the fluid has leaked out of a (damaged) reservoir, the power steering will fail and the car's steering will likely become too heavy to manhandle around corners. This could pose a risk to life and limb.
Maybe you're the sort of muscleman (or musclewoman) who can drive a car originally designed around power steering without any power assistance – but the next prospective buyer of your car will probably draw the line at that.
So while power steering fluid doesn't get the 'mission critical' press of engine coolant or oil, it's still vital that the dealer or independent workshop servicing your car regularly checks the reservoir for fluid that has become too dark – a sign that it needs to be flushed and replenished. This will be in line with whatever the service schedule for your car dictates. At the very least, mechanics should be checking the fluid level during each service.