I agree that the way a pipe thread (tapered) works it should seal without the need of any sealant. To say that Teflon thread sealer is not a sealer but a lubricant only is completely false.
You would be wrong. Very wrong.
The threads cut on the pipe will not match what it's threaded into exactly.
Different manufacturers, different dies, different die wear, different machines different tolerances etc.....mean that the threads on one piece are not exact to any other.
Teflon tape is used to reduce friction and metal to metal galling during the installation.
Brass, steel, aluminium. Dissimilar metals the friction level during installation rises rapidly when threaded together.
Most plumbing tests have a question. "Is Teflon tape a sealer or a lubricant?" If you say sealer you'll get it wrong.
When I went to engineering school there was a similar question.
Teflon btw is Polytetrafluoroethylene. It's a lubricant. Has one of the lowest coefficient of friction of any solid.
It is NOT a sealant.