I feel that idioms make a language more interesting and vibrant. I think mastering idioms means you're using and understanding the language more like a native speaker would. The tough part about mastering idioms is that knowing all the individual words may not help you to understand the meaning of the phrase. For example, knowing the meanings of ace and hole may not help you to understand the meaning of "An ace in the hole" (a hidden strength or advantage). I think the best way to learn idioms is probably to immerse yourself into popular culture to have more exposure to them, and ask a native speaker what they mean.