I see this in other games too, and I call it the 'speedrunner tool'.

Normal players (me) get stomped by a boss a few times until they figure it out and can beat it without the buff (fire paper, lightning grease, etc); Maybe they use them, but then they run out and are never used again...

Some players might use the buff and get an easier win; but if they don't, then they are wasted and are hard or impossible to replace, so players learn to not rely on them.

Speedrunners use lightning grease to kill the boss in 3 hits and are already halfway to the next boss before the buff runs out and already picked up the right buff item for that next boss. When your victory is all but guaranteed, the item makes it faster - otherwise it's not really useful.

I think Ethers in FF games as a trope kind of fit here too. Too valuable to waste, but if you can do it without them, then you didn't need it unless you absolutely needed to kill that boss without using a tent or resting, etc, to save time.