It isn't a wheel alignment shim, it's a brake rotor shim.
Look, the two friction surfaces of the brake disc have to be parallel to each other, right? If they're not, then the rotor is thicker in some places than in others and you get than nasty brake pulsation thing going on.
The planes of the disc surfaces must also be perpendicular to the axis of rotation. If they're too far out from perpendicular you have lateral runout (LRO) which is bad. LRO will make the pads rub one surface most at one point and the other surface most 180 degrees from there, but at the two positions 90 degrees off get less rubbing. Those first two positions that rub the most wear faster and become thinner, and this gives you the warped rotor that shakes your brake pedal and/or steering wheel. This is more of a problem with floating calipers than fixed calipers.
Now, a hat type (slip on) rotor (as opposed to a rotor and hub) is machined when new and may be machined again as part of brake surface. If an on-car type lathe isn't used then any difference in mounting on the lathe and mounting on the vehicle will put the disc out of perpendicular and cause the LRO. In this situation, a dial indicator can be used to measure the LRO as installed, and a shim placed between the mounting surface of the rotor and the hub to bring the disc into perpendicular. This eliminates (or significantly reduces) the LRO and makes it so the rotors don't become "warped."
