Teachers used to recommend that learners use idioms in their writing as they thought that idioms would demonstrate that their learners were well educated. Remember that idioms are commonly used phrases or groups of words that are used figuratively. For example, if you say, that something is “costs an arm and a leg”, you mean that it is very expensive – not that you literally have to chop off pieces of body to pay. It’s often easier to understand an idiom if you know the circumstances in which it is being used.
Idioms can make language more colourful and interesting, but when the same idioms are used too often they become boring and repetitive. Also, if a writer or speaker uses too many idioms together, they will just confuse the reader or listener. So teachers now do not recommend that young writers use more than one idiom. They should rather use their own fresh new comparisons, rather than these old ones that in many ways have lost their meaning.