Adam J. Weigold authored Jul 02, 2013 and Brad King committed Jul 19, 2013

WiX provides a lot of functionality for installers that cannot be
supported (easily) in the default WIX.template.in file.

For most users, the default template should be fine.  However if users
want to produce merge modules, include custom actions, etc, this new
option allows for a hook into how the wxs is produced.