Tourniquets
are indicated to stop bleeding when the patient's life is in danger due to blood loss.
It is, however, not recommended to go wily-nilly applying tourniquets. Please let the professionals do it, if at all possible. This is one of the many reasons why everyone should take a beginner's first aid course.
Note:
I was a corpsman and 8404. In general, Superunknown_GP is correct, tourniquets are expected to cause the loss of the limb. This is, however, not a good reason to let someone die.