Data Manager allows you to easily add, edit, or delete your Qlik Sense application data sources using a visual interface. It also allows you to associate your data either automatically or explicitly. 

Data Manager automatically generates the script that is then executed when the application is reloaded. One big flaw of Data Manager is that you cannot edit the generated script. If you do, Data Manager will not respond to your changes and from that point on, you will have to use the script editor. Qlik will hopefully make the script editor and Data Manager work hand in hand. I enjoy dragging and dropping files and automatically associating tables but the ready will most probably enjoy more having the ability to change or improve the script to their needs.