When the USA collapses it could become the new global reserve currency (it won't but it could)

Point is, it's a currency you can use right now independently from the increasingly unstable-looking US dollar.

You could also use euros, yuan, rupees, or Australian dollars but it's really hard to get an account in those currencies if you don't live in those countries. Crypto is much easier to get access to.

We can create various use case scenarios, that‘s what has been done since 2009, but there is no signs that’s something we are moving towards and so far the only use cases that stuck are the ones I mentioned. For a global currency you would need something stable and while the US is declining, in comparison cryptocurrencies (ignoring stablecoins) are a nightmare of instability