Why do most showers have 1/2 BSP-P threads?
BSP-Parallel threads work best with a sealing washer, like you get on washing machine connectors / taps / hoses and on garden taps / Hozelock fittings. ( These are usually 3/4" BSP-Parallel ).
My new Grohe shower valve has 1/2" BSP-P female threads cut into the casting. And from my googling, this is fairly normal.
To connect these to normal pipework, you need to use 1/2" BSP male ( AKA Male Iron ) to 15mm fittings. But to get these adapters to fit into the shower valve in a watertight way, you need to ***really*** pile on the PTFE tape. ( There's no way to use a sealing washer in the shower valve casting. )
Now, this just seems a bit crap to me. You really shouldn't need to lard on PTFE tape like this to make a connection watertight. Any design which relies on larded-on gloop or PTFE is a broken design.
IMHO, if you are relying on the thread itself as the pressure seal, the design should be using a Tapered Thread, not a Parallel one. That's what tapered threads are for! ( Or use a regular compression fitting, or have a Male BSP spigot to which you can fit a female-with-washer like washing machines have, or any other decent method of connection. )
It seems to me that the use of a parallel thread in this way is not the best.