Idioms make a language sound very creative, and can also get others to think and figure out what the person who is using the idiom might mean.
Using the Literal language (as in saying things directly without any creativity) sounds very common, and it seems a bit boring when they are used in story books.
Idioms in a context makes the context seem very nice and creative.
Often the good writers use idiomatic expressions, and sometimes they have a different meaning. It's mostly a challenging thing, and using idioms in your Language essays will earn you more marks and a better impression, so learning idioms is very important.
People appreciate creativity, and using idioms in your writing an help you with your language skills, and also help you understand the meaning when someone else uses them in their speech.