Gaskets form a mechanical seal which fills the space between two or more mating surfaces, to prevent leakage from or into the joined objects while under compression. The amount of force or torque needed to compress the gasket between two flanges is specified by the gasket manufacturer. When that torque is properly applied a good safe seal is achieved. That is why a good torque wrench is needed.
For more information on the kinds and proper use of torque wrenches click here to read a Fluid Sealing Associations Article on this subject.