They can help a lot, since older sites often have drawings that no longer match what is in the field after years of modifications. Laser scans or photos from the field show what is actually there, and comparing them to the model highlights where the drawings are wrong.
The practical approach is to use scans to check and correct the areas that matter most, like where work is planned or where drawings are known to be poor, rather than scanning everything at once. Any correction should still go back into the official drawings, so the model and the documents stay consistent.