Performance.
The application can provide an interface for the user to edit / change settings that can be saved. There is no API to write to app.config and nothing needs to be reloaded if it is changed at run time. Thus, the settings were created. They provide one place to store applications and user preferences. The default values ββfor user parameters are stored in app.config, which the user can edit before starting the application; but if the application provides an interface for editing / changing certain parameters, they can be saved back to disk in the user.settings file, local to the user, allowing users to have independent and safe settings
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