They wouldn't. A lot of cryptocurrency justifications hinge on made up scenarios. They also ignore the fact that it costs money to even send or spend most cryptocurrency, or the fact that it must be liquidated in 99.9% of spending scenarios, and ignore the challenges in buying or selling cryptocurrency in the first place.

Most exchanges require several forms of ID and verification to buy or sell cryptocurrency. I had to use a smartphone to take a picture of myself with my photo ID before I could even use most exchanges. Someone who doesn't have photo ID, computer or a working smartphone are out of luck in that scenario.

Bitcoin might make sense for remittance, with the intention that it is then liquidated after being received.

Governments with unstable currencies sometimes impose restrictions on the ability to buy and sell them, and bitcoin makes it easier to evade these.

Why would someone give euro against a more risky local money?

hence my question, what does btc change?

I can think of a few things.

It's less regulated so it's more likely people will make poor trades - maybe it's not in their interest as you are implying, but they do it anyway.

Countries can make owning foreign currency illegal. BTC is easier to hide than Euros (cash or electronic). You can trade BTC back to cash without a paper trail by writing down a private key.

In terms of why someone would sell BTC for a failing currency? Maybe they are also a local and have expenses to pay. Otherwise, it's likely the seller is selling for USD or something and there's an intermediate conversion so someone else who needs it ends up with the local currency. It's pretty common to get out of the currency as much as possible (assets, other currencies, BTC) then only dip back in for as little as possible to pay local expenses.