Film matters because film is us. We as a society use the filmic form to tell the stories about who we are. Films are a record of what makes us human and what concerns us in the everyday.
Asking:"why do we need cinema?" is like asking"why do we need to eat?", it just seems to be a need fulfilled - a need to emotionally experience adventures happening on screen. Film works so well because it can expand our reality and take us into its world without almost any effort on our part.
Places where we used to watch films might soon totally switch from cinemas to soft home couches, but film is a wonderful phenomenon that allows us to understand cultures and people, and as long as understanding will matter, the film will matter too.