Why English is one of the most important languages to learn?

Because it's the language of the superpowers out there

1. English is commonly spoken throughout much of the world due to Great Britian’s expansion during the colonial age. People in Australia, New Zealand, Canada, parts of Africa, India, and many smaller island nations speak English. English is the commonly adopted second language in Germany, Norway, Sweden, Denmark and the Netherlands. Speaking English opens these countries and cultures up to you let alone the fact that English is the official language of Science. To excel in science you simply must know English.

If a Japanese and a Saudi Arabian businessmen meet to discuss a business affair, they will choose English as a means of communication

it is universally spoken. 

In recent years international business mediation, relations and contractual agreements have been written in English - and most companies therefore require their staff to be acquainted with, if not competent in, the language.