Merge page allows you to see the differences between the app on your Builder and app on the base or “original” Builder. 

This is useful when you take a copy of an app and customize it, yet from time to time you still want to incorporate updates from the original project as it is being developed. 

You can choose whether to keep your version, choose the original from the base Builder or ignore the difference and show it as resolved in the future. 

The topic is well described here.