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If your talking about the cap that goes over the end of the shaft on the power steering pump & where the edge of the pulley is-my guess is its to prevent rust build-up on the power steering shaft making pulley replacement easier.Example-look at lug nuts & studs on a car that does not have wheel covers/hub caps & one that does.The car with wheel covers/hub caps does not have rust(or a very small amount) on the lug nuts & studs compared to cars that are missing or do not have wheel covers/hub caps which get very rusty & are hard to get off.That cap could also be for appearance.Thats my edumacated guesses
If your talking about the cap that goes over the end of the shaft on the power steering pump & where the edge of the pulley is-my guess is its to prevent rust build-up on the power steering shaft making pulley replacement easier.Example-look at lug nuts & studs on a car that does not have wheel covers/hub caps & one that does.The car with wheel covers/hub caps does not have rust(or a very small amount) on the lug nuts & studs compared to cars that are missing or do not have wheel covers/hub caps which get very rusty & are hard to get off.That cap could also be for appearance.Thats my edumacated guesses
I agree with 94. Plus it looks a little better than the pulley hub. Any maybe helps protect from scratches. But when it boils down to it, its a useless piece of plastic. Just like lugnut covers.