One of the major features introduced with Android 4.2 Jelly Bean is the Multi User feature. This feature allows a person preferences to be saved and loaded when one tablet is used by more than one people. This is very handy where one tablet is shared by a family. Especially a family with kids.
By having each individual to have their own set of preferences to be saved and loaded when they logged in, accidental mishaps can be avoided. When the tablet is used by adults for work, it is safe to know that when their children use the tablet (mostly for playing games) no work related action can be performed by the children, such as sending emails to all the person in the address book, which can be embarassing.
This feature is only available for Android tablet. It is assumed that a smartphone is used by only one person, while a tablet is shared. Therefore, it is unlikely that this feature will make it to the Android smartphone.
For details on Multi User feature, check out the embedded video below :