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.
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 can 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
Learning other languages gives you access to a greater range of information about your subject and enables you to communicate with students and researchers from other countries. If much of the information and research about a subject that interests you is in other languages, learning those languages will be very useful. For example, if you're interested in minority languages of Brazil, you will probably find most information about them is in Portuguese.
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 very useful.
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.
Revitalising or reviving your language
If you speak an endangered language, or your parents or grandparents do/did, learning that language and passing it on to your children could help to revitalise or revive it.
Maybe you're interested in the literature, poetry, films, TV programs, music or some other aspect of the culture of people who speak a particular language and want to learn their language in order to gain a better understanding of their culture.
Missionaries and other religious types learn languages in order to spread their message. In fact, missionairies have played a major role in documenting languages and devising writing systems for many of them. Others learn the language(s) in which the holy books of their religion were originally written to gain a better understanding of them. For example, Christians might learn Hebrew, Aramaic and Biblical Greek; Muslims might learn Classical Arabic, and Buddhists might learn Sanskrit.
Perhaps you enjoy the food and/or drink of a particular country or region and make regular trips there, or the recipe books you want to use are only available in a foreign language
Maybe you're interested in linguistic aspects of a particular language and decide to learn it in order to understand them better.
Maybe you enjoy the challenge of learning foreign languages or of learning a particularly difficult language.
Sounds/looks good to me
Perhaps you just like the sound of a particular language when it's spoken or sung. Or you find the written form of a language attractive. If you like singing, learning songs in other languages can be interesting, challenging and enjoyable.
One language is never enough!
If like me you're a bit of a linguaphile / glossophile / linguaholic or whatever you call someone who is fascinated by languages and enjoys learning them, then one language is never enough.