Learning a foreign language takes time and dedication. The reasons below may help to convince you to take the plunge, if such persuasion is needed. Some reasons are practical, some aspirational, some intellectual and others sentimental, but whatever your reasons, having a clear idea of why you're learning a language can help to motivate you in your studies.
Kolik jazyků znáš, tolikrát jsi člověkem. You live a new life for every new language you speak. If you know only one language, you live only once. (Czech proverb)
When you move to a different country or region, learning the local language will help you to communicate and integrate with the local community. Even if many of the locals speak your language, for example if your L1 is English and you move to the Netherlands, it's still worth your while learning the local language. Doing so will demonstrate your interest in and commitment to the new country.
Family and friends
If your partner, in-laws, relatives or friends speak a different language, learning that language will help you to communicate with them. It will also give you a better understanding of their culture and way of thinking.
If your work involves regular contact with speakers of foreign languages, being able to talk to them in their own languages will help you to communicate with them. It may also help you to make sales and to negotiate and secure contracts. Knowledge of foreign languages may also increase your chances of finding a new job, getting a promotion or a transfer overseas, or of going on foreign business trips.
Many English-speaking business people don't bother to learn other languages because they believe that most of the people they do business with in foreign countries can speak English, and if they don't speak English, interpreters can be used. The lack of foreign language knowledge puts the English speakers at a disadvantage. In meetings, for example, the people on the other side can discuss things amongst themselves in their own language without the English speakers understanding, and using interpreters slows everything down. In any socialising after the meetings, the locals will probably feel more comfortable using their own language rather than English.
Study or research
You may find that information about subjects you're interested in is published mainly in a foreign language. Learning that language will give you access to the material and enable you to communicate with fellow students and researchers in the field.
Language is the archives of history. (Ralph Waldo Emerson)
Many English speakers seem to believe that wherever you go on holiday you can get by speaking English, so there's no point in learning any other languages. If people don't understand you all you have to do is speak slowly and turn up the volume. You can more or less get away with this, as long as you stick to popular tourist resorts and hotels where you can usually find someone who speaks English. However, if you want to venture beyond such places, to get to know the locals, to read signs, menus, etc, knowing the local language is necessary.
Americans who travel abroad for the first time are often shocked to discover that, despite all the progress that has been made in the last 30 years, many foreign people still speak in foreign languages. (Dave Barry)
A basic ability in a foreign language will help you to 'get by', i.e. to order food and drink, find your way around, buy tickets, etc. If you have a more advanced knowledge of the language, you can have real conversations with the people you meet, which can be very interesting and will add a new dimension to your holiday.
Die Grenzen meiner Sprache bedeuten die Grenzen meiner Welt. The limits of my language are the limits of my universe. (Ludwig Wittgenstein)
If you plan to study at a foreign university, college or school, you'll need a good knowledge of the local language, unless the course you want to study is taught through the medium of your L1. Your institution will probably provide preparatory courses to improve your language skills and continuing support throughout your main course.
If you and some of your relatives, friends or colleagues speak a language that few people understand, you can talk freely in public without fear of anyone eavesdropping, and/or you can keep any written material secret. Speakers of such Native American languages as Navajo, Choctaw and Cheyenne served as radio operators, know as Code Talkers, to keep communications secret during both World Wars. Welsh speakers played a similar role during the Bosnian War.
You may be required to study a particular language at school, college or university.
Getting in touch with your roots
If your family spoke a particular language in the past you might want to learn it and possibly teach it to your children. It could also be useful if you are research your family tree and some of the documents you find are written in a language foreign to you.