Intonation is important because it gives English its rhythm. It is the emphasis that is placed on some words and not others. The extra emphasis placed on the nouns, main verbs, adjectives, and adverbs makes them stand out and if you only hear these words you will still have some idea about what the person is saying. This is done in a few ways, but the most common is to elongate (make longer) the main vowel sound in the word. This emphasis not only produces the rhythm, but it also helps in comprehending what a person has said.
If you only hear only “Sally…gone…store…buy…apples…oranges…coke”, you get that ideas that Sally is at the store buying some things. If the intonation (emphasis) is wrong you may only hear “has…to the…buy…some…and”, you will not know what the person is talking about. The full sentence is “Sally has gone to the store to buy some apples, oranges, and coke.”
From this we can see that intonation is important. One way to improve your intonation and put the emphasis on the right word is to practice. A good method is to take an audio or video file for which you have the transcript. First, review the transcript and identify the nouns, main verbs, adjectives, and adverbs. Then, listen to the audio or video file and check to see if you have found all the words that will be emphasized. You can now practice speaking along with the person in the file and put the same emphasis on the same words. Repeat this until your intonation matches with the person from the file.
You can go here to view my course description – https://johns-online-english.com/business-english-course/
For more information and some helpful exercises you can check – http://www.manythings.org/e/pronunciation.html