You need what you need.
I have a few Datsun 521 trucks as a project. The engine is metric. The body is SAE. The pipe fittings on the engine are BSP (British Standard Pipe) Brake line flare nuts are 3/8-24, and there brake line is 3/16.
With most cars, you need both. With more modern cars, you will also need TORX or other odd type sockets/ or drivers. And then when you get the TORX drivers, you will find you need the security TORX drivers.