Such dictionaries are essentially designed to improve the learner's vocabulary and to help them in constructing their own sentences. The power of it lies in its complete and clear definition of an entry. They are seen as the sources of the 'best' language and provide an authoritative guide to usage. The publishing houses created special teams to compile and write a dictionary. This effort will result in considerable revenue.
Publishing houses invest large sums of money, employ many experts and spend money on advertisements.
English Language Teaching is a very important market which is gradually developing by innovations in teaching methods and text books. Learners need reference books by which they can solve a problem they face while reading a text or writing an essay. International Learner's Dictionaries are of great value for learners to refer to. In these overseas markets, there are numerous dictionaries for a variety of audiences ranging from beginners to advanced learners. The four major dictionaries which are widely known around the world are published by Collins, Oxford University Press, Cambridge University Press and Longman, and include invaluable information which most of the learners are unaware of.