I think at the time the designers designed a zoom lens based on intuitions and knowledge/witchcraft, then had to make calculations to verify/understand lens behaviour. With zoom lenses (zoom proper, lenses that perform at all focal lenghts) these calculations take a huge amount of time, considering the great number of lenses and the very great number of "positions" (zoom, focus, aperture), and computer helped make this calculation allowing the designers to figure out much faster at which focal length a problem might have appeared, modifying the design, calculating again etc. This "trial and error" procedure was certainly much faster with computers.