You use these from the apps tab when you want to launch a disposable -
that is the usual way you use them. The disposable is a single use qube
which will disappear when the main window is closed.
You may want to run the disposable template from the templates tab when
you want to set some thing that will affect all disposables based on it.
This could be some configuration setting, or creating some help file,
or similar. The change you make in the disposable template will affect
all the disposables.
I never presume to speak for the Qubes team.
When I comment in the Forum I speak for myself.