An analogy can be drawn to bicycle helmets. A bicyclist isn't supposed to fall on his head, and if all is right won't, so a hlemet is just a waste. OTOH, crashes happen, so the helmets role is to mitigate injury resulting from something that shouldn't happen in the first place.

Likewise with spoke protectors. A properly adjusted RD won't ever go into the spokes and the protector is unneeded. However all that needs to happen is for a fall or minor crash bend the hanger in a little bit, and the rider not notice (or is more common these days correct the problem by adjusting the trim). At some point with the limits ow mis-adjusted, the RD will overshift into the wheel. The spoke protector doesn't prevent this, but it can certainly reduce the potential damage.

Do you need a spoke protector, certainly not. But, if you're not well attuned to your bike's condition, it can save you big dollars in possible damage.