The GetHiddenRuntimeParameter is very similar to the GetRuntimeParameter. Let me provide you a use case for this....
Lets say you create a custom action that creates a form with a number of variables and forms. Using GetRuntimeParameter would allow you to pull the data from a work item and populate the fields as needed.
If you use GetHiddenRuntimeParameter, then the values will still be pulled and populate the variable in the form but the field will not be visible in the form and so will not be editable.
Hope this makes sense.