Someone thought iphone is tough enough, so they just let it naked. Well, that'a an wrong idea. Take iphone 4 as an example: it's made of glass meterial, which means it is tough, but sometimes it's also easy to break. You can find a lot of pictures of broken iphones in Google. So you better get a case or at least a bumper to protect your iphone.

Besides, the battery of iphone cannot (or it's hard to) be replaced on your own, but sometimes the battery drains really fast. So a backup battery case or other external battery would be great.

Resource: Cheap iPhone 4 Charge/ Case/ USB Cable/ Headset, Earphone, Handfree | Dragonext.com