The Events / Plays tab is an additional filter that can be applied on top of the others. It is used to select whether only tasks, performances, volunteer activities, or projects attached to particular plays/events will be seen in the calendar.
If Show only these Plays is selected, then the additional filtering is applied. By default, All Plays are shown.
If you turn off All Plays, you will be able to select individual plays/events so that only calendar items attached to those plays/events are displayed.