I gave an Elm talk years ago at the shared office space I used to work at. A few devs there later told me the talk helped them better understand React.
Of course Elm is closely related to Haskell, which is a playground for category theory. I think learning the why behind it all can be useful in subtle ways.
Elm is a pure functional language where all data is immutable and functions are pure meaning they are guaranteed to have no side effects.
Having no side effects in JS is easy (just don't do it!) but immutability takes some effort.
React requires immutability so that if it sees the reference to an object again, it knows that it contains the same data. If it promised to work when mutating objects it would continuously need to deep search inside them to see what changed.
In JS, some array operations mutate the array, some copy it, you have to know specifically what operation you are using. In Elm, nothing mutates objects. All built in functions and functions you create will not do this.
In short - you can do (state, action) => state in any programming language, but mistakes caused by mutations are impossible in Elm by design.
