The difference is that one is freely useable without worry that you'll have permanently wasted it, and the other weighs heavily on your mind that you've wasted it permanently.
Hoarding behavior in video games has a lot do with players feeling like they've found a key for a special lock, but not knowing where that lock is. And part of the blame for that can be laid on game designers for not putting any appropriate locks in. There's a reason why, though: limited, wastable resources being keys to anything means players can soft-lock themselves out of content.
Limited, wastable resources make quite a bit more intuitive sense in, say, Roguelikes, where runs are short and death is frequent enough that having a special, death-saving antidote as a key does actually fit the problem of you dying constantly.
There's also the added effect that these resources are powerful, meaning that if you did save them for a boss fight or something, you might feel like you were skipping the fight instead of using your wits to overcome a difficulty. It lacks satisfaction.
All that said, I don't really agree. Recognizing that these resources are, by nature, superfluous and counterintuitively low-value means that I should use them flippantly, and that has actually made them way more fun.
[edit] Actually, one more point: You can think of limited, wastable, powerful resources as having a kind of negative inflation. That is, they become more valuable the longer you hold onto them. If they get you past a difficult fight, well, the next fight will be more difficult, so shouldn't you use it there? Much like with real currencies, this creates a system in which people are punished for spending, so they won't spend.