It is important to develop your vocabulary. As well as helping you in English communication generally, a good range of vocabulary is essential for the IELTS test. It’s going to help you immensely in the Reading section of the test where a lot of academic language can be used, and it will help you in the listening where synonyms are often used. Most of all though you’ll see the benefit in your writing and speaking results.
So, if it’s a good idea to use new vocabulary in your writing and speaking when you practice, it’s also a good idea to use it in the test, right? WRONG!