To get a good score in IELTS for Lexical Resource (LR), which is IELTS speak for vocabulary, you need to demonstrate an awareness of style and collocation. At least that's what it says in the LR descriptor at Band 7.
Co (together) + location (location!) = Collocation
Sometimes - in fact quite often - 2 or more words appear together, but not for any grammatical reason. They just do!
For example, which of the following do we never say?
We never say Merry Birthday. It's grammatically correct, and it's communicative, but we never say it.
Another example would be verbs that go together with nouns. Here is an example from Bahasa Indonesia.
In Bahasa Indonesia, the verb make collocates strongly with the noun homework.
In English the verb-noun collocation is different:
Low band IELTS candidates tend to translate directly to English from their first language, and often the result is weak collocation.
High band candidates tend to think in English, and as a result they have in their minds chunks of language that include strong collocation.
For example, in the sentence..
But some, such as pregnant people and those with weakened immune systems, are at greater risk of getting sick from animals.
..there are at least 6 chunks:
Mostly these chunks are acquired through reading. Indeed, regular reading is the best way to acquire strong collocation.
Go do some reading!