Ad extensions helps to show the additional information on Google AdWords ads. Extension can make your ad stand out more. Which gets you more valuable user interactions like clicks, call and store visit. It expands the reach to customers providing more information about the business.
There is no cost to add extensions to your campaign. Clicks on your ad will be charged as usual. So you are charged for a click when someone calls you from a call extension or when someone downloads your app from an app extension.