For most people, this question is easily answerable, but there's always that slightly smaller mass of guys and girls speaking an important international language that wonder why they need to learn another one, when they can easily handle themselves about with the help of their mother tongue. Some of the reasons to learn a foreign language that I'll utter here might seem a bit too obvious, whereas others could spark an interest, regardless, if they determine you to take up on a foreign language then I'll be a happy man :).

Moving abroad

Obviously, if you've already moved abroad then you will absolutely have to learn the local language if you want to be able to blend in with the society. If you're expecting to move abroad in the near (or far) future you should take up on language lessons as soon as possible and not wait until the last second.

Career bonus

Learning a foreign language can be such a huge career boost in many occasions that it could prove to be an extremely useful investment of time and money. Conducting business in a foreign country will be eased up if you know the local language and it also shows a higher level of bonding and respect from your side, possibly allowing for a better professional relationship.

Traveling

If you're reading this, you probably have at least a moderate grasp of the English language and you might be wondering why you need to learn a foreign language while traveling, since, let's face it, you can get by with just English close to everywhere in the World. Well that's a wrong assumption to start with actually and it's also a bit disrespectful to wait for them to speak English on their home soil instead of you trying to learn their language. Whether you're visiting a foreign country for 2 weeks or 2 months, at least knowing the basics will help a lot.

Cultural reasons

Yes... cultural reasons. You might be interested in a different culture's history, poetry, music or prose and let's face it, translations in these cases aren't the most accurate. Learning the language of the culture you're interested in is a great way to study it up close and understand the details that would've been left out in a translation. Knowing two or more languages will also boost your cultural status amongst you fellow colleagues, co-workers or friends, since a polyglot has always been highly regarded in any era's society.

Although these are the main reasons that most people are learning a foreign language for, there's a dozen more that we could specify here. Having a foreign spouse, getting a scholarship in a foreign country or simply learning out of fascination for another language can all be serious reasons for the subject at hand.