Idioms in a programming language are a sort of lingua franca to let future readers know exactly what we're trying to accomplish. We may document our code extensively, write exhaustive unit tests, and hold code reviews three times a day, but the fact remains: when someone else needs to make changes, the code is king. If that someone is you, all the documentation in the world won't help you understand unreadable code. After all, how can you even be sure the code is doing what the documentation says?
We're usually reading someone else's code because there's a problem. But idiomatic code helps here, too. Even if it's wrong, when code is written idiomatically, it's far easier to spot bugs. Idiomatic code reduces the cognitive load on the reader. After learning a language's idioms, you'll spend less time wondering ``Wait, why are they using a named tuple there and more time understanding what the code actually does.
Using a given languages set of idioms makes it easier for other developers to read and understand your code. Furthermore, if other people use the same set of idioms, it's easier for YOU to understand their code!