Its the only way to be certain that the vehicle doesn't level downwards if someone gets in and out of the vehicle. My vehicle probably has millions of faults stored from my "investigations" so they'd have to be cleared before looking at anything.
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The only reason they would actually get into the vehicle while doing an alignment, is to centre the steering wheel. However this is generally done by leaning in the window. An alignment is completed with the vehicle in Drive, the handbrake on, and the brake pedal locked down. Any forward/back movement of the vehicle during the alignment process, and the operator will have to start again. ( resweep ) A door ajar stops the suspension from moving. The only niggle I have experienced, is the ignition turning itself off. The older setup with a key was better. The ignition needs to be on to allow free movement of the steering wheel for the sweeps.
Hope this helps.