In the OP's case, the _why_ can be encoded in function names. For example:

    function DatePicker() {
      function handleDateChange(event) {
        preventDialogFoobarFromClosing(event);
        ...
      }

      return (
        <ThirdPartyDatePicker
          onChange={handleDateChange}
        />
      );
    }

    function preventDialogFoobarFromClosing(event) {
      event.preventDefault();
    }