If I have a set of checkboxes or a select/multi-select box that happens to have a large list of items, it would be convenient to the front-end user to be able to “select all” or “deselect all” choices for just that filter.
For instance, if I have a table set up to not display until the user makes a filter choice from a long list, it would be convenient to allow them to “select all” if that’s what they were looking for. Or if they had started making several individual choices, have the ability to “just clear that filter’s selections”.