Max Kreminski:

Monads are a solution to a specific problem: the problem of repetitive code. If you write enough code in a functional programming language, you start to notice that you’re writing a lot of suspiciously similar code to solve a bunch of superficially different problems. Wouldn’t it be nice if you could just write this code once and then reuse it, instead of rewriting it slightly differently every time? I’m omitting a lot of detail here, but this is effectively what monads allow you to do.