Why is an Interactive Service account needed for NPrinting server service?

If the service account used to run NPrinting (16.x/17.x/18.x/19.x) is not Interactive (does not have "Log on as a service" rights) and cannot logon locally, tasks that connect to a QlikView document (.qvw) are expected to fail.
In NPrinting 16.5 is was also found that a non-interactive service account cannot run external XML requests either.

Environment:

• NPrinting all versions
• QlikView Desktop all versions
• Qlik Sense Enterprise all versions

When connection to QlikView (NPrinting 16.x/17.x/18.x/19.x):

The reason why the NPrinting service must be an interactive user is due to the fact that during the reports generation process, QlikView Desktop is used to get the required data from the qvw documents. QlikView Desktop is not a service but a executable application with a graphical user interface (GUI) that needs a interactive user to work.​

When using NPrinting 17.X and 18.X with only Qlik Sense connections, these are the requirements:
(We will refer to the "Service User" as the user running the NPrinting Engine Service)

• The Service User must be RootAdmin in Sense
• The Service User must be member of the Local Administrators group
• The Service User does not require to be Interactive.

More information under Windows Qlik NPrinting Server services administrator