Some do it for their career (because it's required) and some do it for "fun". I speak5 languages and willing later on to learn a6th one. It always fascinated me. You are at ease when you travel to that country and speak their language...and they feel more at ease too. But the greatest appeal of all is "adaptability" I would say, which nowadays is quite difficult.
Aforeign language is a window through which one can look and discover the unseen details of other civilizations' experiences that could be different and rich in certain domains.