It started because of the colonies and the Britain sailors that sailed around in the world. It has grown because of the colonies and because of people in the world found out it was an easier way to communicate, because of this it also has started to be 2nd learnt language in many countries. Many of the words has been created by old historical people(like Shakespeare) and poets. The English language is not the same all over the world, because of the different countries have different accents and some of the countries have different words for the same things, even USA and Great Britain have different words. It's more people in South Asia that can speak English than in USA and Britain together.
History of Global English in 10 Minutes
How English Became a Global Language