Babies learn by babbling which phonemes they need to communicate in the language that people around them use. They then get rid of sounds they don't need, while learning to put two sounds together, then put words together, and so on until they are completely fluent in the language. Some might say that phonetics is the basis of language, it's certainly one of the smallest elements of speech.
Different languages have different sounds altogether in their languages that you don't have the ability to produce because you've never needed to in order to communicate.
Idk, it's quite interesting when you think about it. It's not like from day to day to need to know exactly what phonemes you're producing and how or why, but it can be very important for things like speech and language therapy, learning or teaching new languages, aiding child language acquisition, etc.