Oh, ok. If you want to design a language nowadays, knowing monads (and other abstractions) is absolutely necessary, even if just to reject them in an informed fashion.
If you want to use a language to program, knowing them will help you with approximately nothing, because most languages either have awful ergonomics for that kind of abstraction or bring them on much more limited ways that you are better learning how they specifically apply to the language than the general concept. If you learn the general concept, the specific cases will be easier, but unless you plan on learning several languages, it's not worth it.
Anyway, one large exception is if you plan on doing heavily abstract C# libraries. The C# support for monads is almost as general as Haskell.
If you want to use a language to program, knowing them will help you with approximately nothing, because most languages either have awful ergonomics for that kind of abstraction or bring them on much more limited ways that you are better learning how they specifically apply to the language than the general concept. If you learn the general concept, the specific cases will be easier, but unless you plan on learning several languages, it's not worth it.
Anyway, one large exception is if you plan on doing heavily abstract C# libraries. The C# support for monads is almost as general as Haskell.