An action parameter field can either show an edit field or a variable popup selector. You toggle between these using the Variable toggle. UseVariable is the state of that Variable toggle.
Some actions set UseVariable to true in Reset to force the action editor to show the variable popup selector, with None selected, instead of an edit field. This can help make choices more intuitive or reduce clutter, e.g., if the default choice should be None rather than the default value for that parameter type.
Does that make sense?