All but the simplest apps need to store data. The Stocks app lets users track their favorite stocks, so it needs to store the list of stocks to follow that the user chose. Each time the user launches the Stocks app again, it displays the list of stocks the user inputted previously. If the user adds or deletes stocks from this list, the Stocks app needs to store this updated list and retrieve it again the next time the user loads the Stocks app.
Other types of apps may have various settings that allow users to customize an app such as defining its background color or sounds ...
