In the pre-app world applications could do real damage to your machine. They ran as your user, and they did actions on the machine on your behalf.
Apps are different. They aren't trusted to do actions on your behalf, and they must be sandboxed to prevent damage to your machine.
Apps are not new - not on PCs or elsewhere. We've had flash games for forever on PCs and apps have always been the main way of interacting with smartphones.
The difference is that on PCs you've never been able to install native programs that genuinely can't do damage to your machine. They can't install other software, they can be installed and uninstalled without litterring your machine, and they must declare upfront manifests of how they intend to operate.
iOS, Android and Windows8 are good examples of post-app OSes. Windows7 and to a lesser degree, Linux and MacOSX are a pre-app OSes because native code you found on the Internet for them is dangerous, unattributable, non-selfdescribing, non-centralized and does not necessarily cleanly uninstall.