The IELTS public band descriptors for speaking state that in order to achieve band 7 or above for Lexical Resource (LR - vocabulary), then you should use some idiomatic language.
Idiomatic language gets you a high score for vocabulary because idiomatic phrases contain strong collocation. Take for example:
I eat nasi padang once in a blue moon.
Like any other idiom, the words - once in a blue moon - always appear in exactly the same form and in exactly the same order. The words collocate strongly with each other.
We never say:
There is a lot of teaching material available online dealing with idioms, but there are very few opportunities to use the commonly taught idioms in IELTS speaking and writing.
A blue moon occurs once every 2.5 years, so if you eat nasi padang no more than once every two years, then ok. Otherwise, if you eat nasi padang once every few months, then saying once in a blue moon would not be appropriate.
If you use an idiom, it's a good idea to explain what you mean. For example, if you use think outside the box, do it like this:
If you cannot do this, then you probably don't know what the idiom means and you probably shouldn't be using it.
In a separate post you will find flashcards filled with idioms suitable for use in IELTS Speaking, including the idiom featured in the image at the top of this page. Test yourself and your friends - How many of these could you already use with confidence?