Have you ever looked up a word in the dictionary and then had to look up three or four more words in order to understand the definition? This is why these publishers have written learners’ dictionaries.
All of the definitions are carefully written using clear, simple language so they can be easily understood by people whose first language isn’t English. Each dictionary has a unique style with their own advantages. Visit their home pages and look at the different activities and exercises they have to get a feeling of which dictionary you like best.