I've never had one in my hands, but I would guess these were introduced to try to prevent road debris from building up around the boots of the lower ball joints. I wouldn't say it is "cosmetic". It certainly looks like they could shield the boots from flying debris. From the front of the car, with the dust covers installed, you can't even see the boots. It seems reasonable this would result in less rocks and junk pelting your boots.

Personally, I left them off because I just liked the idea of the higher torque bolts (which I understood to be too short to accommodate the dust covers). It could be that the dust covers themselves can't tolerate the higher torque, so that may be why there is a lower torque spec to begin with.

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