Idioms are very useful because there are some situations or ideas that can be addressed succinctly while technically every idea can be given without using them at all. They reflect cultural or historical backgrounds and thus often connote certain emotions that are shared by same language speakers. = with the same language. Not only can you understand idioms only if you already knew them, = Even though you know them already there still moments that you can' t understand it, but your expressing power will be optimized by exploiting idioms properly. = that will help you to really understand it. However, you must keep in mind that the true purpose of a language is to communicate with others, to understand them and to be understood by them. Consequently, idioms are useful only when all people engaged in a conversation are aware of them. = because they became aware to it. For instance, I observe = observed that Americans refrain themselves from using advanced words, phrase verbs, = phrasal verbs, or idioms when they talk with other people who are speaking English as a second language when = whenever I go to an international conference or workshop. Note that all non-natives speakers there (except me probably) can speak English perfectly but since their main concerns aren't English itself, they wouldn't bother to spend huge amount of time to learn idioms. In this case, idioms only make the speaker not be understood. To wrap it up, idioms are good to know but you should be considerate = aware when to use them.