Of course, there are a lot of useful things a pure functional programming language gets you, like Haskell's implicit IO scheduling and threading, that Rust doesn't. But for many use cases where functional programming languages are great, they're great for specific reasons that Rust is also great at.
Also having actual purity knowledge (something Rust punted on before 1.0) can sometimes be useful. Although honestly 99% of the time it's only for the benefit of the compiler, which isn't a big deal if you have the tools to write code that's efficient from the get-go.
e.g. list fusion is enabled by purity, but is largely uninteresting in Rust because lazy iterator chaining already orders operations and avoids intermediate lists just like list fusion.
It's solving the same problems, and the solution ends up having many similarities with functional programming.