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