Safety. If you don't have restrictions, you get something like Windows/os x. Everyone can install anything they want from any source and that is also what makes Windows/os x much more vulnerable then Android or iOS. Priveleged apps are special applications with more permissions then normal apps, for example they can access the sd-card, while normal (web-installed) applications cannot. For more information you could take a look at: https://developer.mozilla.org/en-US/docs/Web/Apps/App_permis...