Why be fluent in English?
That’s the first thing that comes to our mind. Whether you like it or not, English is the de facto language of business in many countries and for cross-border communication, and interviews for most meaningful jobs are conducted in English.
And its importance is only going to increase in future (point # 3 further down in the post).
I don’t need to convince you on importance of speaking fluent English for professional success. You would have observed this around you and probably experienced too.