The IELTS assessment criteria says that to get a Band 7 for Coherence and Cohesion, you need to use “a range of cohesive devices appropriately”. Using carrier nouns will help you to increase this range. Remember, most test takers tend to reply on transition signals, referencing and conjunctions for cohesion. If you also include some carrier nouns, your range will increase.
Even better, using carrier nouns will also increase the range of your vocabulary, helping you achieve a Band 7 for Lexical Resource.
It also makes your writing more concise because you are replacing a lot of words with one word.
Good essays use clear, concise language, not long-winded, repetitive language!
Note: using carrier nouns does not guarantee Band 7! You must use them appropriately – in other words, don’t try to force them into your writing – just use them when they’re needed.