A Quickform node does not only create a flow variable. It also creates a small GUI.
Usually wrapped metanodes do not have a configuration window, unless they contain a Quickform node. In this case, the wrapped metanode acquires a configuration window, where the parameter(s) of the internal Quickform node(s) is/are exposed to change.
The same GUI is automatically generated when executing the workflow, and therefore the wrapped metanode, on the KNIME WebPortal.
You can see all of this in practice in the following video.
The workflow shown in this video can be found on the EXAMPLES server under 06_Control_Structures/02_Quickforms/01_Quickforms_and_Metanodes06_Control_Structures/02_Quickforms/01_Quickforms_and_Metanodes*.
* The link will open the workflow directly in KNIME Analytics Platform (requirements: Windows; KNIME Analytics Platform must be installed with the Installer version 3.2.0 or higher)