[turdherder]

Real life licensed and certified plumber here.

Regarding the threads on common pipe fittings here in Cali: NPT threads are designed to be cut with a pitch and angle that are self sealing.

Sealing compounds can assist in the lubrication of threads to easily tighten them up but should not be absolutely necessary.

Imo it's an aid to assembly and disassembly and not always necessary depending upon the application. And in some applications it's forbidden