Another purpose for idioms and idiomatic speech relates to bonding and forming communities with people. Idioms tend to be informal, social language, and their use "warms up" a social situation, helping you bond with other people as "like" yourself. A person unable to use the idioms feels-- and sometimes is-- excluded from the bond. I think this helps account for the coinage of new idioms or figurative vocabulary among young people. It gives them their own "inside talk" that adults can't share in... without looking stupid :)
Idioms can be used to deliberately exclude from conversations a person who speaks the language well, but not natively, or they can be used to manipulate or make fun of the non-native speaker. (Don't feel paranoid, English learners. I don't think it's common, but it's absolutely possible in English and in your language too.)