When should an advertiser use an app extension instead of an app promotion ad?

Q) When should an advertiser use an app extension instead of an app promotion ad?

Ans:

With keywords intended to drive app download, but still give the option to visit the mobile website

With keywords intended to direct users to the mobile website, but still give the option to download the app

To re-engage with users who have already downloaded the app

To run your ad on both Search and Display