Idioms are typically used informally; hence, they shouldn't be used in the IELTS writing test. Using them in the speaking test is acceptable because spoken English is typically much less formal than academic English.
Idioms and quotations could be included primarily in the Speaking and Writing modules because they go well with the lines that are needed for each of these modules.
It might add some depth and make the examiner find your response more compelling. The quotes must, however, be appropriately presented and should flow into your essay or speaking response.
Conquer your fears and use as many idioms and collocations as you can. Your English will sound much more fluid and natural after doing this. Do as much speaking practise as you can, ideally with native speakers.
Phrasal verbs and idioms are employed to convey facts, concepts, or emotions subtly in a phrase or term that has a secondary meaning.
Your vocabulary and lexical resources will increase as you get more familiar with idioms and phrasal verbs. Your band score will subsequently rise as a result.