This would typically be used if you had the POPUPS disabled in the ADMIN setting, and a module loaded a secondary VIEW or EDIT control using the ControlKey parameters.
/PageName/ctl/View2/
As a module developer you would do this by having multiple controls defined in a module definition.
It could also be used even if you have Popups enabled, for controls that are marked to not support popups (defined in the module's definition).