[iggldiggl]

Airplanes might be designed in mm, but airports are most likely designed in metres, and whoops, suddenly you need to convert between the two.

Actually civil engineering as a whole is probably a relatively mixed bunch – vehicles drawings (when you need to design the infrastructure to fit specific vehicles for example) and other small-scale details might be dimensioned in mm, the standard curbstone naming system in Germany is based on centimetres, while infrastructure design itself (definitely including the CAD drawings) usually works in metres.