Over the last few centuries English has evolved very rapidly. Therefore even the speech, that belongs to the modern stage of English, such as the works of William Shakespeare might seem difficult for us.But nowdays english expressions are frequently used in most of scientific articles and works. Moreover it became one of the working language of UN since 1946.
The most usual reason , why people start learning English is the global spread of English. English is the official language for the air communication and relations in all international communal spaces. In addition it facilitates the debats in scientific and political communities.
Many people try to master in language by learning it abroad. I believe it is straight way to learn foreign language.Also it gives you a chance to practice and develop your oral skills.
To conclude with, although, i think the fact, that English aids people to communicate with each other, it also has some shortcomings. For instance the fast growth of using English may lead to disapperance of minoroty languages in North America and Australia.