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Why does a "rapid" preheating mode require no more than one oven rack?

I have a Whirlpool wall oven with (optional) convection settings.  It has three racks.  The obvious place to store oven racks is, of course, in the oven.

The oven has two preheating modes, "rapid" and "standard".  The documentation says to use the "rapid" mode with only one rack (removing the other two) and the "standard" mode otherwise.  Both modes run the fan audibly.

Why does the presence of more or fewer racks make a difference in *preheating*?  I've found explanations online about using only one rack to improve circulation when cooking -- sure, that's plausible, but we're talking about *preheating* here, not *cooking*.  Doesn't it use a thermostat to determine when the oven is up to temperature?

I would naturally prefer faster preheating to slower, but I want to keep all the racks in the oven.  What can go wrong in using the "rapid" mode with all racks in place?