Well, that's an interesting question. I'm not sure if you mean the language or the course, so I'll answer for both:
Why was English (as a language) created? Language exists as a means for humans to communicate with one another beyond the concrete and the immediate; any language gives its speakers to ability to communicate figuratively, abstractly, and with some sense of spatiality and temporality. English was created as a combination of Germanic, Romantic, and Celtic roots.
Why was English (as a course) created? English courses have several goals. For non-English speakers, this course allows them to develop a second or foreign language, an important skill in today's global society. For native English speakers, the course allows for the development of an academic understanding of the language, which is different from the colloquial knowledge that speakers pick up from their context. The grammar, punctuation, spelling, and vocabulary of the academic and professional worlds are different from those of daily life. English also allows students to be exposed to elements of culture deemed "classic" by certain people, such as writing techniques and reading material. The canon of texts to read and skills to cultivate is by no means impartial and has significantly racist and classist roots, but continues to thrive.