I'm an engineer, and I also regularly buy plasterboard/wood and other things. Yes I agree while everything in the housing trade is measured in mm, you still need to know about imperial measurements - and quite often need to work in imperial if you're working with tools/fittings that require it.Go and buy a sheet of ply in any builders merchants
1220 x 2440 x 19 or 8' x 4' by 3/4" in old money
Example: In the offshore industry - and rigs that are owned & built by US companies - you could use for example 3/4" pipe with NPT threads, and lots of the nuts and bolts are in imperial. Look at drill pipe sizes and torque settings measured in lb/ft.
Perhaps in UK/Europe most sizes are in metric - but you still need to know both if you are any sort of engineer.
Even maths uses it - example, what's 1/2 of 3/4? need I say more.
When you buy oil - sold by the Barrel - 42 U.S. gallons.
In speech - you don't generally say 'what's 50% of 10?', or 'do you want 50% of an orange?', any normal person would say 'Do you want half of this orange?'.