Why do I need to use Whitworth hardware?

A little about the lathe set up to cut Whitworth threads and why I need to use proper Whitworth BSF and BSW threaded hardware for this Riley car build. This difference in bolts and tools isn't actually obvious unless you have come across it. And this is my layman's explanation. I refer here to British and US threads but things get complicated by the fact that 50s and 60s British cars used the US UNF and UNC hardware too. So my MGB has mainly UNF/UNC threaded hardware but some Whitworth. It gets more complicated when you take into account some of the British Pipe threads. And BA threads.... It all just means that for someone like me with pre-war British cars, Post war British cars and modern metric machines I just need a hell of a lot of different spanners, sockets and taps and dies!