By default popups are allowed unless there is a filter to block them. When this setting is turned on, all popups will be unconditionally blocked for the current site, regardless of filters
As seen in issue #1143, this wouldn't be a good idea. Blocking all popups is a last resort step when usual means (the filter lists) do not work on a specific site.
From wiki:
Can we have a global setting to turn this on/off?