People learn English for many different purposes.
First of all, person who speaks English relatively free has much more opportunities to find a good job in the foreign company. He can work as an interpreter, translating some negotiations, buisness letters, broshures and advertisement.
One who knows English properly can be sent to the training session at the head office abroad. He can communicate with English spoken partners face to face and reach some agreement much faster, than someone who speaks with a translator.
Secondly, it is a great opportunity to enhance your qualification reading some special technical materials from internet. There are plenty of good articles, electronic books, video tutorials in English on internet. You can share your experience with colleagues from another countries working in the same area as you do. Nowadays, a lot of new ideas and technologies come to our life from abroad. People who know English have an access to news first.
Thirdly, English spoken people have less problem travelling abroad. It goes without saying that English has become an international language. The tickets booking, hotel reservation, dinner in the restaurant is much easrer if one can speak English.
The last but not least, is some opportunities for students to study and live abroad, make friends, get some helpful work and communicative experience.