A situation where tanks are not at full health and engaging against a multitude of damage dealers, including AT nades and infantry AT weapons.
In my experience it is very rare for 1 tank to go up against 1 tank destroyer and slug it out, usually there's a bunch of stuff going on and with multiple units firing.
Even if you assume that a snare or shrek has already done 80 damage, again the extra 40 damage from the jp4 doesn't actually improve time to kill. Since iirc the only 120 damage increment that axis has is the puna, in that specific situation the 40 extra damage would be a benifit.
But of course there could be possible situation, the extra 40 damage may be a benifit in specific circumstances, but discussions about its performance "in a vacuum " discussions like the ones that are occurring in this thread, the extra 40 damage by the numbers doesn't provide any benifit.