They can be used to help learners with pronunciation.
Example:
The following are examples in English:
/p/ as in pet /k/ as in cat /ei/ as in pay /i:/ as in beat
In the classroom
Activities which can help learners include phonemic bingo, phonemic hangman, practice with dictionaries, class posters and looking at homophones. There is much discussion about how useful phonemic symbols are to learners, and some teachers choose never to teach them, especially if learner do not feel they are useful.
See also:
https://www.teachingenglish.org.uk/article/phoneme
https://www.teachingenglish.org.uk/article/phonemic-script
https://www.teachingenglish.org.uk/article/phonology
Further links:
https://www.teachingenglish.org.uk/article/phonemic-chart
https://www.teachingenglish.org.uk/article/phonemic-symbols
https://www.teachingenglish.org.uk/article/young-learners-phonemic-chart
https://www.teachingenglish.org.uk/article/teaching-pronunciation-phonemic-symbols
https://www.teachingenglish.org.uk/article/vocabulary-recycling-revision-phonemic-symbols
https://www.teachingenglish.org.uk/article/introducing-phonemic-alphabet
https://www.teachingenglish.org.uk/article/remembering-phonemes
Good resource.