If a single programming language is and will be enough for your work and communications, then knowing more languages may be useless. But it does and is very useful if you need to. Languages today are evolving all time.
Take JavaScript for example, it's learning features from all kinds of programming languages and in the early days, due to the lack of guides, reading guides on other languages can be helpful. Even in some cases, you need to do it by your own to learn skills in another language and do it in JavaScript. That's really touch story since you need to learn concepts and terminologies in various kinds of languages, just to get it right.
In short, more languages will help you know the precent of the discoveries in the history, and help you foresee the future of a single language.