[twic]

particularly with any trade related measurements

This is the key thing, i think. There is a convention in some technical disciplines - mechanical engineering, architecture, civil engineering up to a point - that things are measured in millimetres even when they're on the scale of metres. So you get a ceiling that is 2400 mm high, or pilings on a 1500 mm spacing, etc.

I'm not sure why. I suppose it means that the large things are directly comparable with small things that are more naturally measured in millimetres (if your tiles are 300 mm, you need eight courses for your 2400 mm bathroom wall), and you can use single unit on drawings taking in large and small things (the engine block is 900 mm long, the bolts are 12 mm across, on 150 mm spacings).

It probably ties in with preferred numbers somehow:

https://en.wikipedia.org/wiki/Preferred_number