Why do these Doctor Who devices count as doomsday weapons?

Why do the Crucible, Reality Bomb, and Demat Gun count as doomsday systems in Doctor Who?

The Demat Gun removes a target from time and space, while the Crucible carries a bomb with a much wider effect through time.

Within Doctor Who, the Demat Gun removes a target from time and space. The Crucible deploys the Reality Bomb, whose threatened effect reaches far beyond one target. Destruction through all time is treated as the ultimate weapon in a setting of Time Lords and time wars.