Tapered pipe threads, NPT and others, were designed to seal without sealers. The catch is that works ONCE, for the initial tightening but not for subsequent uses, and requires accurate, good quality machining.
In most cases that is not the case. Even if a new fitting is used the threads it screws into may have been altered by the removal of the first fitting so that seal cannot happen.
Sealers are used to fill the tiny gaps between threads that almost always occur. Many fluids may not leak as they are too viscous but many will, including gasses.
Plumbers and anyone working with fittings,, pneumatics, hydraulics, gas fitters, use sealers routinely because the consequences of a leak from one lousy fitting can be major rework and very expensive. Not just the cost of the part but the rework time , the mess, the lost production and all the associated costs.
There are places that sealant should not be needed such as JIC, compression, flares unless they have been damaged in which case they actually should be replaced but where those fittings are adapting to a NPT a sealer is almost always used. In the case of the flares, JIC, compression, sometimes the damage is minor enough and pressure low enough that a sealer will work. I have used some Loctites in those cases, small quantity just to fill the gap, not gobs. However unless darn positive of no leaks after, it was replacement time.
I should mention that many fittings use either an O ring for the seal or a crush washer that should be replaced each time that fitting is disturbed. The actual threads are straight, not tapered. Many, many of those types of fittings are used industrially.