The filter widget offers a "clear all" feature. You can set "All fields values" as a default values. So lets say you have 20 possible field values. The user opens the filters, all the fields values are pre-selected. He just wants one field value to be selected. He clicks on "Clear all" then clicks onthe field he wants. Fine. But now he wants to get back to all field values selected. He has to click 19 fields... A "select all" is absolutely essential for this filter widget.