Not all pipe threads are self sealing - only a tapered is - NPT and NFT. Self sealing also means they have to be tightened almost to yield, and removal and reuse may not allow self sealing without tightening deeper while in some cases deeper (thus tighter interference fit) may not be possible. Should note JIG, AN, ORB and flair fittings are self seal also.
Thread seal and tape are not just for lubrication when non self sealing threads are employed - they actually take up the space between the threads and the root.
Further - there are specific applications for different thread sealants, as mentioned. Natural/lpg needs special tape (normally coded yellow just like gas line). Not all pastes can hold against petroleum products so check before using. Some Rectorseal formulas can, others can not.
Tape should not be used in any fuel system connection or any critical system connection as improper application can lead to blockage of the system. similarly when sealing threads in polymer items (like my well pump for example) tape must be used because the chemicals in the sealant may affect the polymer and cause it to break down.
Proper installation of tape is critical to performance and most failures of a joint sealed with tape are caused by incorrect use. Tape should never be applied to the end most 2 threads to prevent it from shifting and blocking flow through the passage. Tape should also not be applied more than 3-4 turns around the male end, and must be wrapped in opposite to the direction of the threads. In other words, normal right hand threads require CW rotation of the fitting to tighten. Tape must be wrapped CCW so the turning of the fitting holds the tape in place where wrapping in the wrong direction would unravel the tape.
I've done countless connections with dopes and tape with several liquid and gaseous contents on multiple applications (including medical devices) with very, very few failures when the above is followed.
Good stuff, use it for most every connection.