English is the language of today. It is normally the common means of communication in today's world. The English speaking nations, today are leading the world and therefore, in order to be successful in the modern times, one has to be able to communicate with those who lead.
Most of the media today is led by the English language. A major chunk of the World Wide Web is in English. Television is led by English language channels. The best reading material is found in English.
In order to be able to succeed in life today, I think it is utmost necessary to be able to speak fluent English. Even in non English speaking Countries, English is being used as the major language of communication, especially on the professional level. Even other than the professional life, English is increasing becoming the mode of communication.
Since the world is getting smaller and smaller, and as they say its turning into a global village, one has to come into interaction with people from other cultures and countries more often. People from various Countries and even from different regions in the same Country usually do not understand each other's original language. There fore, they have to find a common mode of communication. A language that both parties understand, which most of the time is English. What I am trying to say that in today's world, where inter-cultural and inter-regional communication has become a necessity, the need for a common language cannot be overlooked. English is doing a good job as that common language.
The question that you have asked can have many answers.
You should know how to speak English fluently if you are living in a country where this is the usual language people converse in.It would make your life easier in that country and it would be easier for other people to get you and for you to get people. There are many times when a person who is not fluent in English cannot get his/her point across properly. He might want to say something and it might be interpreted as something else by other people only because the person didn't have the right words and the fluency to be understood properly.
In case you are living in a country where the official language is English but the language mainly used is some other then they might not require you to be fluent in English but if you are, you would be able to land yourself better jobs.
In case you are living in a country where English is neither considered the official language nor is spoken otherwise then you don't really have to worry about not being fluent since it wouldn't matter there.
But generally speaking it is good to be fluent in English since it is used in everyday life in a lot of countries.