Why does my teacher make me learn the phonemic script?

Good reasons why your teacher might teach you the phonemic script

1. To spend more time on pronunciation/ to give variety to pronunciation practice - Most students come to English class with improving their pronunciation as one of the top priorities, but this is something textbooks only spend a little time on and it is difficult to practice outside class. If the teacher is going to work more on your pronunciation that will usually mean spending more classroom time on it, which means you will get bored and so not learn much if you only do drilling. Most of the other activities you can do to improve your pronunciation in class, such as pronunciation games, need you to write down the sounds in some way. The only good way to write down the sounds of English is with the phonemic script.
2. To help you write the pronunciation down - There is no way of representing all the sounds of English clearly using just the alphabet of English or any other country. If you do try to write down what you hear in the alphabet of your own country, that will just make your own non-native accent stronger and make it more difficult to understand people who do not speak the same way as you. Therefore, if you want to write down the pronunciation in your notebook so you can refer to it later in the same way as you write down the meaning of vocabulary and grammar, you must learn the phonemic script.
3. To help you remember the pronunciation - Writing something down is a great way to help you remember it, and the same is true of pronunciation.
4. To teach you how to use a dictionary - If you want to recognize a word you have read and looked up in a dictionary when you hear it later, you will need to learn the pronunciation as well as the spelling and meaning. All good dictionaries give the pronunciation using the phonemic script.
5. To help you study pronunciation outside class - As well as using a dictionary, you also need the phonemic script to use self-study pronunciation books and language laboratories.
6. So you can organize the pronunciations you write down - Like vocabulary, the best way of learning pronunciation is to group things together by similarities. In fact, many people believe that this is how the information is stored in your head. If you understand the phonemic symbols for each word you are learning, you can group vocabulary together by the same sound and so help your pronunciation and your vocabulary learning at the same time. 
7. To show the difference between spelling and pronunciation - The students who complain about English spelling often think of the phonemic script as another type of spelling to learn, but actually it is the best way of making sure you don't get confused by the English spelling when trying to pronounce a word and of practicing different spellings of the same sounds.
8. To show how sounds change when they are put together - When two sounds from different words are next to each other, this often changes the sounds. To give one example, when there are two /t/ sounds next to each other like in "What to do?", one of the /t/ sounds is changed or lost. Studying this point can help with comprehension of natural speech and with fluency of speaking. The only clear way to show such changes is with the phonemic script.
9. To show how sounds change in natural speech - As well as sounds next to each other, the sounds of English change in lots of other ways such as the different pronunciations of "can" when stressed in "YES, i CAN" and unstressed in "I can DO it". The only way of writing these changes is with the phonemic script.
10. To give a good model for drilling - As well as copying the teacher's voice or the tape and the instructions on how to move your mouth, knowing exactly which sound you are trying to produce can help involve other parts of your brain in your efforts to make the right sound.
11. To give information to help you produce the individual sounds - The symbols of the phonemic chart include some easy to understand information about what kind of sounds they are, for example the two symbols that represent the separate parts of each diphthong and the two dots after a long sound. Seeing this written down can help you remember it, and the symbols can also help the teacher explain your pronunciation mistakes quickly and clearly.
12. To show the system behind English sounds - As well as showing long sounds and diphthongs, the phonemic chart also groups together words that have similar sounds and so makes it easier to see the similarities and differences. For example, pairs of voiced and unvoiced sounds are next to each other in the chart.
13. To stop you thinking about the spelling when you are listening - As well as making reading and writing difficult, the irregular spelling of English can also cause problems with listening, for example by making it difficult to recognize a word when you first hear it. Learning the phonemics for every word as well as the spelling can help reduce this problem.
14. To improve your listening comprehension - All pronunciation work that is important for listening comprehension, such as the sound changes in rapid speech mentioned above, are easier to study if you know the phonemic script.
15. To make everyone the same level - Even students who have very similar language levels in speaking and grammar knowledge can have very different accents when they speak. Teaching the phonemic script allows the teacher to go back to the pronunciation basics that the students with more problems need while teaching all the students something new.
16. Because it is difficult to learn on your own - Because learning the phonemic script involves not just getting to know the symbols but also making sure you can say the right sound for each one, lots of help by a teacher is usually needed.
17. To help logical thinkers - People who are naturally good at pronouncing another language include musical people and others who do hobbies that involve carefully controlling their voices and other parts of their body. The mathematical/logical people who find grammar study easier usually need a bit more help with pronunciation, and learning the phonemic script can especially help these kinds of learners and mean they can catch up with the rest of the class.

Bad reasons why your teacher might make you learn the phonemic script

Hopefully the reasons above have convinced you that learning the phonemic script is a good thing, but as with most things in life there is a small chance that your teacher might be doing the right thing for the wrong reasons. If all the reasons above are not relevant to you and you think one of the reasons below is true for your class, you might want to ask your teacher if you could spend less class time on phonemics.

1. To practice it themselves - Even native speakers find the "code" of the phonemic script hard to learn, and just like for you, for the teacher the best way to learn is by using it in class. This doesn't have to be a bad thing if you are learning something at the same time, but might mean that it is used too much. If so, you might want to ask your teacher or the school manager if some of the phonemics work could be given for homework instead.
2. To show off - Once your teacher has learnt the phonemic script, they might enjoy showing they can use it so much that some of the times they do so your pronunciation could be corrected in quicker and easier ways. The best way of showing your teacher that they don't have to use it so much is to learn the phonemic script so well that your teacher can see you don't need any more practice.
3. To fill time - This is probably not the reason why your teacher uses phonemics, as there are much easier ways of filling classroom time! If you think this is the case, the best way of making it finish quicker is to show that you know phonemics very well already.