There are many purposes for studying foreign languages. Some of the more common reasons are to translate one language into another, to work or live in a country that interests you, to attend school in a foreign country, to learn about the different cultures of the world, and finally for personal social reasons.
When I visited Central Asia several years ago, the children approached us to talk with us - to practice their English. Many have told us they speak multiple languages such as English, German, French, and their native tongue. Some also speak some of the Asian languages. They impressed me very much, because I have lived in the US most of my life, and my primary language is English, and I can speak, write, and read a little Japanese.
With language, if you don't use it, you lose it.