The pads are almost always at least partially touching the rotor, just not with any real force. Even if we assume that you manage to knock them away just right (slalom the car a bit before parking to try to induce pad knockback?) to get a tiny gap going on all 8 pads, it's still better to give the brakes a cool-down lap than to come in off a hot lap and park immediately.
The rotor will cool all the way down from hot-lap temps to ambient temps either way. The difference is that the more of the cooling that's done while rotating (cooldown lap) instead of fixed (parked), the more even the cooling is. While parked different areas of the rotor cool at different speeds thanks to the shapes and positions of the caliper, pad, wheel, suspension, etc which will absorb heat at different rates and/or block off ambient wind cooling. Having different parts of the rotor cooling at different rates is going to stress the rotor more than having it cool off evenly while spinning.
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