Using a monolingual dictionary can be hard work, especially if you have to carry it all the way to class yourself! There are lots of good reasons why your teacher might want to train you in using an English-English dictionary. These reasons are listed below: 
 
1.   Stop translating:
If you can switch off L1 (your first language) part of your brain, it will eventually become possible to think in English and so speed up your comprehension and production of the language. Using a monolingual dictionary not only means you are reading in English and avoiding your own language, but that you can also copy the definition down into your notebook and so increase your use of English.

2.   Double practice:
Many students are worried that when they look for one difficult English word in the dictionary they will just find it explained with another difficult word they also don't know. This will rarely happen with the right dictionary and if this happens all the time you should probably be using an easier one such as an Elementary Learners' Dictionary. If this still happens occasionally even when you have the right dictionary, that means that the word you don't know in the definition is also very useful and you are therefore getting double practice of English by learning that one too.

3.   Grading:
As mentioned above, English-English dictionaries are available in many different levels. By using an Intermediate Learners' Dictionary, for example, you will know that any words which you read in a text that do not appear in the dictionary are not important enough to learn at this stage of your English studies and so can be ignored. If there are many words in the article you are reading that are not in the dictionary for your level, that is also a good sign that the text is too difficult and you should read a graded text instead. Bilingual dictionaries are not often graded this way.

 4.   Words that don't translate:
Another problem students have is that they understand the English definition but still can't think what that word is in their own language, and so they go to a bilingual dictionary to check. Usually the translations given in a bilingual dictionary are not really the same thing as the English word. This is particularly true with personality words, where seemingly the same word in different languages might have very different positive and negative meanings. This is another case in which stopping translation helps.

5.   Learning English grammar words:
Students also sometimes complain that words like "noun" and "adverb" in the English-English dictionary make the definitions difficult to understand, but these are words you will also need in order to be able to study English without using your own language. A dictionary is a very good place to learn grammar words from because the same words are repeated many times. Copying these words down (or short versions like "n" or "adv") when you learn new vocabulary can help you learn the vocabulary better and learn the grammar words at the same time. Knowing these words in English will also help you discuss the language with people who do not speak your language, for example if you study abroad.

6.   More information:
A good English-English dictionary for foreign language learners includes lots of extra information to study that most bilingual dictionaries leave out, such as the most used words in the English language, more definitions for each word, common confusions and mistakes, and the phonemic script for pronunciation.
  
7. Learning to explain language in English:
The way that a dictionary explains a word in simple language is very similar to what you will need to do if you can't think of a word in English and have to explain what you mean in other ways. Reading such explanations will make it easier for you to make explanations yourself such as "I can't think of the word, but it is a kind of...”

 8. Quality control:
By using a good English-English dictionary for learners recommended by your teacher, you can avoid the problems that some bilingual dictionaries (especially electronic ones) have such as lots of useless or out of date words no one uses or lack of definition for each word.  

9. Learning usage of words and collocations:

All bilingual dictionaries do not provide any example in which the new word is used. Moreover most of the bilingual dictionaries do not mention any thing about the usage of words. It means that they don’t tell where and how you can use the words. On the other hand an English-English dictionary provides lots of examples and tells you exactly how and where you can use the words. In addition they provide some useful collocations which are the natural combination of words. Example: ‘Beautiful success’ is grammatical but it is not a natural combination in English.  

To reach to an end, although using monolingual dictionaries might be time-consuming at first you will benefit a lot from it. In other words it’s advantages are more than it’s disadvantages.