Research shows that people mentally organize English by stress patterns. It's how they process the sounds they hear. This means that the correct stress, on the right syllable and on the important word, is vital to helping the listener understand the sounds that he or she is hearing.
If your stress, intonation and rhythm are "off," the listener may not understand the word or phrase you used, or may spend a second or two trying to figure out what you said, and miss the rest of your sentence.