Since the release of iOS 7 brings a whole new interface design for iPhone applications, developers might be forced to rework the user-interface of their iPhone apps as well, to match Apple's new Human Interface Guidelines. But even if operating system fragmentation is not too much of a problem in the iOS world, there will be users staying on previous versions of iOS. Regarding the Human Interface Guidelines, apps designed for iOS 7 would be inappropriate for those users.
Up to now, iOS users were only able to download the latest version of an application available in Apple's app store. If an application was updated for a newer version of iOS, and a user with an older version of iOS wanted to download the app, he was simply prompted to upgrade his operating system first. If there was no newer versions of iOS available for his device or the user didn't want to upgrade, there was no way to install the application.
Along with the release of iOS 7, Apple refreshed its iTunes Connect Developer Guide and announced, that it is now possible for users to download previous versions of applications:
Previous versions of your apps are now available for re-download by users who have already purchased them, allowing customers to use your apps with older devices which may no longer be supported by the current version of your app. If you do not wish to make these versions available, you can manage the availability of your apps' previous versions in the Rights and Pricing section of the Manage Your Apps module in iTunes Connect.
For developers, this brings a certain flexibility regarding application updates, supported operation system versions and devices. It's now possible to keep a service alive for devices like iPhone 3G or 3GS which won't get any major iOS updates anymore. At the same time, current versions of an app can make use of new features or user interface elements.