Phonemes.
Phonemes are the sounds made by each letter of a word. In English, each letter is not always pronounced the same way in every word. e.g., rat, rate, wasp and awe. The ‘a’ in each word is not pronounced the same. That is why we need the phonemic alphabet in English.
These are the phonemes that represent the words. /ræt/, /reɪt/, /wɒsp/, /ɔː/
Each word has the letter a, but the a is pronounced differently in each word. Rat has a short a, rate has a long a, wasp is pronounced with a short o sound, and awe is pronounced aww.
That is why the English language needs the phonemic alphabet.
Every word in the English dictionary has a phoneme written after it which tells you how to pronounce it. Online dictionaries also have recordings of how to pronounce each word.