Native speakers intuitively know' which words frequently combine and which do not. To a native speaker, they just do not sound right. Knowing frequent collocations is essential for accurate, natural English. In Lewis's approach, a third category is fixed expressions, and a fourth, semi-fixed expressions. Collocations and expressions are thought to be the most important types of lexical phrases.
Native speakers retain many prefabricated lexical items in their memory.
Language fluency and accuracy is achieved largely by retrieving and combining ready-made chunks of language. In recent years, there has been a worldwide increase in demand for ESP, which is essential for professional development. Lewis's ideas of teaching vocabulary in chunks appealed to us because of its efficacy and have been applied to teaching ESP skills. In ESP, students have to learn high-priority lexis, which needs to be selected and included into learning materials and class activities. Obviously, students do not need to distinguish which category lexical phrases belong to.