Phonetics is the study of the sounds that humans are able to make. Phonology is the study of how those sounds feature in language and human communication.
I studied linguistics at university, and I'm still asking myself why understanding these two fields is important!
In all seriousness though, everything from speech therapy to understanding the way our brains process information can be tied into phonetics and phonology.
But, as to why it's important to language - well that's an interestingly worded question. I've always considered phonetics and phonology to be the study of an aspect of language that would exist in language, whether we studied it or not.
Humans will continue to use their vocal folds to voice sounds until they evolve to communicate in a more efficient way...
I guess the answer is that the phonetic and phonological adaptations that humans have made during their evolution have allowed speech-based language to develop.
Both our vocal organs and our brains have developed in such a way that they allow us to string a series of sounds together, and these sounds represent everything from really basic and intrinsic concepts, to the expression of highly complex thoughts.
But would that need to communicate be met if we weren't able to vocalize sound in the same way? The answer is yes.
Language can exist without phonetics and phonology.
