They are one way in which new words can be created.
Example
'Splash', 'crackle' 'thud' and 'drip' are all examples of onomatopoeia.
In the classroom
Onomatopoeia are an interesting and fun area for learners. As well as learners guessing what they represent, activities which use them include comparisons between languages, writing poems, creating sound effects for radio plays, and inventing new examples. There are many examples in children's poetry which can be exploited.
Further links:
https://www.teachingenglish.org.uk/article/rain
https://www.teachingenglish.org.uk/article/new-words-english