Learning a foreign language can help you to understand your own language better and give you different perspectives on your language and culture.
Learning related languages or languages that have contributed a significant amount of vocabulary to your native language can be very interesting and useful. In the case of English related languages include Dutch, German, Norwegian, Danish and Swedish, and languages from which a lot of English vocabulary comes include French, Latin, Greek, Old Norse, Italian and Spanish. It you learn one of these languages you will find quite a lot of familiar words, which helps a lot, and some of the foreign words you learn will help you understand English words. For example, if you know that ego means I in Latin, words like egocentric become easier to understand, and if you know the Greek words anthropos (mankind) and phobia (fear), the meaning of anthropophobia should be obvious.
Learning foreign languages can also help to understand grammar better. Many people in the UK and some other English-speaking countries are not very familiar with grammatical terms like adjective, adverb and case, but when learning other languages such as German, Russian or Irish, it helps if you understand the significance of these terms. You could learn these languages without overt knowledge of their grammar, but requires extensive and intensive exposure to them, which is what native speakers get. A knowledge of foreign languages can also show you that there are different ways of expressing things, and of structuring language. If you're used to putting adjectives in front of nouns (e.g. a funny story) it can take a while to get used to language that put them after nouns (e.g. une histoire drôle), but once you get used to this, it becomes second nature and this helps when learning other languages - your mind becomes open to other ways of expressing things.
Part of learning a foreign language involves learning about the culture of those who speak it. This can be very interesting, and can also give you a different perspective on your own culture. There are many ways of behaving and interacting that are natural to you and are part of your culture, but when you encounter another culture and are asked to explain your own culture to outsiders, might lead you to consider why things are as they are and whether there are better ways of carrying on.