Why do you think prose is an important element of language?

Prose adds style, pace, and flow. Think about how boring poems would be if they were continuous run-on sentences or if stories consisted of "Bob did this, and this, and then this, and then..." Good prose engulfs the reader and takes ordinary sentences and makes them into art.
Also, prose is the general term for the ordinary language of everyday life and commerce, and it is not in and of itself artistic. There is much more boring or at least fact-based prose than there is artistic prose. It serves as a language's work-horse, as the medium of all the ordinary communication that has to take place all over the world from moment to moment.