The example shows that the emphasized words make a difference to your understanding of what the person has said. From this we can see that placing emphasis correctly is important in speaking understandable English. This extra emphasis on the word is done in a few ways, but the most common is to elongate (make longer) the main vowel sound in the word. This emphasis helps in understanding what the person has said, and it also produces the rhythm of English. This makes Intonation am important part of pronunciation because it makes the important words stand out, it gives a rhythm to the English language, and it affects the pronunciation of the emphasized and non-emphasized words.
Given the importance of intonation it is surprising that some or many ESL teachers don't spend more time stressing intonation when they are teaching English. I see this as a problem because not emphasizing the right word and emphasizing the wrong word can cause comprehension problems for the listener. It can also change the pronunciation of the word such that it sounds like a different word. This is because an emphasized vowel sound can sound different than a non-emphasized vowel sound for the same vowel. It is the difference between the long vowel sounds and the short vowel sounds for a particular vowel.