The majority of US consumer grade ratchet/wrench sets have both US and metric sizes. There are enough metric nuts and bolts in random things that you really do need both.
Need both, in USA - the at home toolbox in SI countries don't have this problem as they rarely bump into the odd-duck US measurements. The US system require to buy more tools - so I can work on most stuff (SI) and the some stuff (USA).
Buying two things to do one job - as is tradition.