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Because they make putting the brakes back together a bit easier.
I dug around and asked mechanics about these a few times. I have found two other possible reasons:
1) To make sure the rotors are not off center since the lug holes are bigger than the studs. This makes no sense on cars with a properly fitted hub that matches the rotor bore.
2) If a wheel starts to come loose, without the screws the rotor can start wobbling, creating a sort of pad knockback effect and when the driver hits the brakes, they can go to the floor. Even if the wheel flew off completely, the rotor could become a spinning thing that destroys the brake line, making this worse.
That last point is likely the most probable thing that could happen. Still a pretty specific edge case with things lining up in a bad way. But I can see some point ot that.
But of course on our cars, for those that go to the track a lot, the odds of cracking a hub because of the screw holes is real, so running the Karcepts hubs eliminates the screws in the rear rotors anyways. But .. track cars.
There are also HOZAN JIS screw drivers available, wich are also great. Not as good as the Vessel, but 90%. I own both, the Vessel and the Hozan.
You could recognise JIS screws mostly by a tiny round punch mark. The great thing about JIS screwdrivers is that they dont cam out. You just twist the screw open, like a hex bolt. There is no need to push the srewdriver towards the screw.
Simply great.
Picture: HOZAN screwdriver stick into the screw of the brake fluid reservoir of my Kawasaki. No tricks, no Magnets, it´s just the precise, lightly clamping fit wich holds the screwdriver upright.
American workers couldn't be relied upon not to overtorque things, which jis allows. Philips is inherently torque limiting, as tool cams out from fastener under load.
Which of course incents US workers to press super hard as they apply torque, training them to apply torque while removing any feel or feedback, exacerbating the problem.
You can replicate the benefit of rotor screw during brake servicing by temp installing a lug nut to hold rotor in place.
American workers couldn't be relied upon not to overtorque things, which jis allows. Philips is inherently torque limiting, as tool cams out from fastener under load.
Which of course incents US workers to press super hard as they apply torque, training them to apply torque while removing any feel or feedback, exacerbating the problem.
You can replicate the benefit of rotor screw during brake servicing by temp installing a lug nut to hold rotor in place.
I replaced many of the soft steel or aluminum screws on my Honda dirt bikes. The ones that I frequently removed for maintenance. Like motor, clutch, suspension. There is a company that had all the replacement hardened steel made-in-Japan JIS screws You just needed to observe torque specs, so as not to crack delicate magnesium-alloy cases.
