It's one and only useful feature compared to ES/TS is the syntax. Once you get used to it, which doesn't take long, you start to experience that in comparison to say ES6, you:

- Write/edit code faster - Read/understand code easier

People say this is subjective, but my experience with other developers has shown otherwise. I once asked a team of developers to "just go along with CoffeeScript for 2 weeks" instead of arguing its syntactic decisions (they were developers who had only used C-style syntax languages in the past like Java/C++/JavaScript). All of them ended up loving CoffeeScript and staying with it for years.

Mind you, TypeScript was a very different language back then. Once it started adding features like disjoint unions and the tooling matured, I made the switch.