For me the appeal at the beginning was the ability to use curve to pay anywhere with Amex and curve promoted this heavily during the early stages. It was fantastic. Shortly after member numbers started to rise, Amex support was pulled. Call me cynical, but I think behind the scenes they were paying the amex merchant fees and netting a loss on some transactions with the idea of then getting everyone hooked on the other features, then to bring in a subscription fee to repay investors. I may have said this on here back when Amex support disappeared.
Curve lost a lot of its appeal as other providers and CCs were offering similar perks (fee free transactions, decent FX rates) so now it's simply being able to use one card and have the aforementioned benefits.
Even that though, is a huge risk fr your USP to be 'carrying just one card' as I've also experienced declines over the years, a handful of odd 'double transactions' and the slow customer service I use it simply for paying for my YT premium, netflix, hosting and a handful of other random annual subs. Most curve users wouldn't ever just carry the one card and thankfully have at least apple pay, if not their wallet, at all times.
Also, I just noticed the iOS app recently updated and isn't going beyond the splash screen now, either.