Some npt fittings are made to seal on the rim of the tapered threads, just like shower hoses using a flat rubber washer does.
One example is Flair-it fittings that use a swivel nut fitting for connecting to male npt threads. They use a seal called a santoprene seal. When the nut is tightened it compresses the seal against the end of the male npt threads and against the fitting.
There are about 6 or 7 of them in my TT. 4 of which I added when adapting my npt threaded captive air tank to the plumbing and a sink sprayer with shut off valve for rinsing skid marks in the toilet.