I can think of at least two real-world uses where cryptocurrency is a better choice than fiat: (1) sending money overseas with very low transaction fee, and (2) transacting on the black market.
Except that fluctuations in the conversion rate make this very touchy, especially if you're talking about a significant sum of money. Also, sending crypto anywhere outside of a handful of developed nations is fraught with difficulty because recipients need to be able to convert bitcoin into spendable money which often involves risky in-person meetups and gigantic markups on the conversion rate.
Also, having options is better than not having them. Sure, there's the international wire system, but it wouldn't hurt to give it some competition.
One day there will be no need to convert to fiat.