I guess I don't really understand the point of it, but what is $1B "worth" of Bitcoin actually useful for? I saw one of the other comments link to daily trading

I guess I don't really understand the point of it, but what is $1B "worth" of Bitcoin actually useful for? I saw one of the other comments link to daily trading volume, but I read those values as consisting of putting a dollar value on Bitcoin movement within the ecosystem, not crossing boundaries - am I correct? So given the practicalities of shifting the value outside of the Bitcoin economy, the main value seems to be in speculating that this specific cryptocurrency is the future and will take on a dominant value in the real world in exchange for goods and labour, and its volatility seems to make that less likely lest everyone turns into the person who "paid X Bitcoin for a pizza on YYYY-MM-DD and now it's worth Y". Am I missing something important here? I'm not an economist, and only have a broad understanding of the technology behind Bitcoin, but I don't see the point unless one day we start saying "$1 buys X bitcoin" and not the other way around. Edit: It's telling that I hit post and then spent minutes obsessing over 'YYYY' and 'Y' being used in same sentence to mean different things :/