The pipe threads are tapered and that is what seals. It is also
why used pipes don't go together as well as new ones.

Is this the same reason why manufacturer recommend us to replace the
gas pipe connecting to a waterheater whenever we replace the
waterheater? (I saw that warning on a package of homeowner waterheater
flexible connecting fittings replacement kit.) Does this mean that the
threads of such gas pipe fitting becomes deformed when it is being
screwed into another fitting?

Jay Chan