While a tourniquet can be used to stop catestropihic bloodloss, it rarely is used for this and probably should be used less than it is. Here's why:
If you have a bleeing forearm, a tourniquet proximal to the wound can be cranked down to stop ALL bloodflow. The good news is the bleeding has stopped. The bad news is all the blood distal to the tourniquet is quickly deoxygenating, and the blood and the tissue it was feeding is now in the process of dying. So are agents of the immune system in that area. And, as there's an open wound, the risk if infection is high, and increases ever second.
There quickly comes a point where the contents of the forearm below the tourniquet are quite septic. At this time, releasing the tourniquet floods a compromised body with a surprisingly heavy bacterial and viral load as well as a quantity of deoxygenated blood withi will oxygenate very quckly indeed, which is clearly undesirable. The althernative is amputation.
If there were no alternative, we could discuss this in seupulchral tones and figure we're saving a life at the cost of a limb. But we aren't.
Nearly ANY wound on an extremity can be controlled by direct pressure. This includes guillotine amputations and a variety of other horrors.
I imagine that somewhere out there, there's a wound so bad that I'd need to tourniquet it, but then I'd have to wonder if even a tourniquet would be enough? At that rarified point, there may be no good answer.
In short, once the staple of battlefield medicine, the tourniquet is rarely used in a first aid context except for venipuncture nowadays.