1) The British Empire, which spanned the globe from the late 16th to the early 20th century, was the largest empire in history and made English a truly international language. Today it is the first language of about 400 million people in 53 countries, and the second language of as many as 1.4 billion more. English was therefore well positioned to become the default language of science in the wake of the disruptive wars of the first half of the 20th century.
2) The biggest library in the world is at least 50% English. Most of the content produced on the internet (50%) is in English. So knowing English will allow you access to an incredible amount of information which may not be otherwise available!
3) Major countries like the USA, UK, Canada, Australia, New Zealand have good economies therefore English is needed to communicate with people from those countries.
4) After the U.S.A's victory over Nazi Germany, Europe as a whole started speaking more English German and Russian but English out paced them all due to western influence
5) English acts as a medium for all countries in the EU.