They have a brush inside so you can apply to the surface of the rotor before installing the caliper. This allows the assembly to slide on easier and the pads to break in properly. Its break in lube and rotor lubricant all in one bottle.
Be better off putting some emoji's on that post, lest someone read it, try it, and spear something with a 2 ton car. (No, putting grease on a rotor surface is not good, in any way.)