Intonation is a huge part of English communication and it has nothing to do with whether your lip, tongue or jaw shape is correct.
Depending on whether there is a rising or falling pitch change at the end of your sentence can determine whether you sound uncertain, are asking a question, said a statement or sound confident.
Being aware and in control of HOW you speak allows you to control how you want the interaction to happen. Do you want the listener to respond? Do you want the listener to provide their opinion? Do you want to communicate that something is factual and not an opinion?
All of these things can be communicated through intonation.
Imagine your boss asking you if you can complete a project.
Because of your rising intonation, he hears, “Yes I can?”
If it sounds like a question, your boss cannot be confident you can do the job. He may understand the words you said but the intonation pattern communicates uncertainty and does not instill confidence.