Teflon tape is a lubricant, it's not meant to seal the joints. So for example if you're screwing a shower head "hockey stick" pipe into a drop ear elbow in the wall, and it stops at a weird angle instead of pointing down -- use PTFE tape on the threads to make them slide against the ones in the fitting more easily, it'll turn further.

To seal a NPT-threaded joint, use pipe dope (it's both a lubricant and a sealant).