Why make an Android when you have a perfectly good iPhone? Why make a printing press when you have a pen? Why make a car when you have legs? Why make a television when you have a book?
New technologies, and new ideas are what keep the world advancing. Yes, the iPhone exists. I personally have one. I know that it is not the optimum, but it does what I need it to do. I'm what they call, "Behind with the times" and I'm comfortable there. However, that means nothing. Progress is constantly happening. Before the iPhone, we all had flip phones. Before that, house phones. Your question, about ten years ago, would be the equivalent of asking why do we need a cell phone when we have a perfectly good house phone?
In ten years down the road, the iPhone will be obsolete and forgotten; a relic. You cannot be mad at the future, because the future always wins. There is nothing that you can say or do to prevent that. Accept and embrace the change, keep what you like (I like my dusty iPhone), and accept that you will never be completely caught up with everything that is happening in the world. If you don't, you are sure to only bring anger to yourself.