Idioms are words, phrases, or expressions that are either grammatically unusual, as in, “Long time, no see!”, or their meaning cannot be taken literally, as in, “It’s raining cats and dogs!”
This expression does not mean that cats and dogs are falling from the sky, but it is a metaphorical expression (word picture) that means that it is raining very heavily.
He cried crocodile tears because he wanted his dad to buy him something.
Just as a crocodile cannot cry, the boy was not crying at all! He was just acting!
People use idioms to make their language richer and more colorful. Idioms and idiomatic expressions can be more precise than literal words, often using fewer words but saying more.