It is the most widely spoken language in the world, Americans, Britishers, Australians speak English. Most of the Europeans know English, In India, Pakistan, China, Japan fair amount of people know English. Many people may not be able to speak English well but they can understand and write English fairly well.
English is the medium of instruction in most of the good school across the world. In India, China Japan, Thailand, Singapore, Malaysia, in the Middle East, in most of the schools where the elite go or people who want to go for higher education, go for English schools. Therefore it is important to learn English.
In the US where most of the research and higher education of good quality exists the medium of instruction is English.
For trade and commerce English is necessary, most companies all over the world follow the English law to settle disputes.
All in all it is important to know English, since it is a world language, most widely spoken and understood, it is therefore imperative to learn English.