It takes much time to learn a language. There are many reasons to learn a language.
Emigration
When you move to a different country or region, learning the local language will help you to communicate with the local community. Even if many local people speak your language, it's important to learn their language. It will also give you a better understanding of their culture and way of thinking.
Work
If your work is about languages, talking to them in their own languages will help you to communicate with them. Knowing 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. The lack of foreign language knowledge puts the English speakers at a disadvantage.
Travel
When you go on holiday you can get more information by speaking English. If people don't understand you, all you have to do is speak slowly and turn up the voice.
Food
Perhaps you enjoy the food and drink of a country or region and make trips there. It is very useful to learn a foreign language.