Some things should really only be used when there is maximum return in terms effectiveness. This is a sensible approach to make the most of your resources. No point wasting a fireball on a few goblins, when it could just as easily kill a vampire.

It depends a bit on what your view of "effectiveness" is, though. Why waste your fireball killing a vampire when you could use it to make an insanely cool interjection at a meeting of the town elders to decide whether or not to pay your adventuring group for ridding the town of goblins?

Transcript

C-3PO: You remember the Lost Orb. I told you all about it and the Peace Moon.
Luke: Oh, that. Can you play back the entire cut scene?
Leia: You have the Lost Orb?
R2-D2: Yep.
[SFX]: < whir >
Leia: The power was literally inside you the whole time?
R2-D2: Uh-huh.
[SFX]: < whir >
Leia: You could have used it at any time?
R2-D2: No! Only when it was maximally—
[SFX]: < boop biwhree bippity >
GM: Oh, before you go on, Pete, read this note.
{beat}
R2-D2: Awesome.
[SFX]: < bee-oop >
R2-D2: I'm ready to rock this ship and roll!
[SFX]: < doop dip-deep boobleep bing eeroo-biddle >
Chewbacca: I'll... get some extra duct tape.