Rightly or wrongly, the way that you speak is one of the first things that people notice about you. Do you know that people will judge your background, education and social standing just by the way you speak?
Even if you can already communicate well in English if you do not speak clearly enough, people around you will not understand you and they might not even want to talk to you. Why? Because it is just too tiring to try to understand you!
Listening and understanding go hand in hand. If you do not know how different words are pronounced you will not improve your understanding of English.