When we learn a language, we learn to express ourselves in that language, using the expressions particular to that language. The way we express things (ideas, needs, emotions, among others) in one language may not necessarily be the same in the language we are learning, e.g. English. That is why it it is so important to learn the common, or stock, expressions, or ways of saying things, in a language. This applies to learning English as an additional language or as a lingua franca (a common, or 'bridging' language).