If you live in a situation where the only local fiat currency is even worse, then the “volatility fee” of cryptocurrency may be well worth the money to decouple yourself from that bad fiat currency risk. Maybe you can purchase things directly with the cryptocurrency, or maybe you just wait until you can drive across the border to a place with more stable fiat currency, sell cryptocurrency for that currency, and buy milk. Simple currency exchange or traditional custodial stores of value like precious metals wouldn’t work, since you’re still beholden to the original fiat currency problem.

At any given time, some small slice of world currency demand will be focused on this “protection from unstable fiat currency” and some other small slice would be like a prepper mindset hedging their perceived risk of facing that (eg “but what if the US dollar collapses?” from people with very extreme risk aversion preferences).

That total “market cap” demand for cryptocurrency will surely be way, way lower than traditional fiat currencies, but it may still be much higher than the current total market cap for crypto, which means there is room for completely rational speculative buying and holding of crypto, depending on your outlook and risk preferences, for perfectly sound reasons.

I highly doubt Bitcoin is ever going to become a major direct medium of exchange. But also it does not need to become that. It can offer big value in a lesser role that would still justify speculative buying of crypto today.