As almost all machining, this does not have one simple answer that covers every situation.
Forming taps can be preferred when tapping blind holes because essentially there are no chips.
Forming taps can also be preferred when tapping "softer" metals because the forming (actually extruding) action somewhat work hardens the metal making stronger threads.
Forming taps apply more cutting forces to the material and to the tool, which can lead to tool breakage. Forming taps generally take more horsepower because of this.
Cutting taps (what you call ordinary taps) make chips. Chips can bind the tap causing breakage.
Forming taps make more consistent threads than cutting taps.