Phonetics, simplified at its core, is the study of speech sounds. In other words, it is studying the way the sounds of the English language are produced; how to pronounce them correctly. Now, you might be asking yourself why it is important in the process of learning English as a second language.
First of all, it is important for your pronunciation. Indeed, because phonetics studies the way sounds are made, it helps students who have some understanding of this particular field to notice when they pronounce certain sounds wrong and correct such mistakes.
Second of all, because it helps students make the correct sounds, it allows them to be understood better too. Sometimes, making the wrong sound in a word may lead to a misunderstanding that could have been avoided if the person pronounced the proper phoneme (word used to describe the smallest unit of sound).
Last of all, it gives a better understanding of the language as a whole. For example, it might make students understand why a word is spelled in a certain way.