Problem:
How to display all Results of a Parametric Search View, and have the possibility to set a Filter Item that "displays all" results?
Solution:
Views does load all results of a Query when you load the Page where you insert the Parametric Search and Results by default.
Then, after you begin to filter by your parametric search, the results returned are the ones that match your Filters.
If you want the default value (which is the "no filters" value), have a Label "All", you can do that when inserting the Parametric Search Filter.
(it's the "Default value:" options)
This only works (is only available) for the Input Type "Select", "Multi-Select", "Checkboxes", "Checkbox" and "Radio"
Those options are all available in the Wizard, when you insert Parametric Search Filters