Here's why:
* Idioms are expressions whose meaning is not deducible from the individual words that make them up. They function as a single unit with a meaning that is different from the sum of its parts.
* They are often metaphorical or figurative. For example, "kick the bucket" doesn't literally mean to kick a bucket; it means to die.
* Idioms are specific to a language or culture. What might be an idiom in one language might not be in another.
Examples of idioms:
* "Spill the beans" - to reveal a secret.
* "Break a leg" - good luck.
* "See eye to eye" - to agree.
* "Once in a blue moon" - rarely.
Understanding idioms requires cultural knowledge and familiarity with the language they are used in. They can be challenging for language learners, but they are also an important part of mastering a language.