Behaviour of the system under differing conditions can then be studied and predictions made. 

The accuracy of the results depends on the quality of these rules...and how well they represent real life.

• there is no danger to humans
• extreme situations which may not occur very often may be studied
• they can be done in a controlled environment (eg computer room) 
• it may be used repeatedly as often as needed.

• reality is complex and only an approximation of reality can be achieved.
• expensive hardware may be needed (eg flight simulator)

Used for training pilots to fly. Difficult and dangerous flying conditions can be simulated (turbulence, storms, fog, lightning etc).

Pilots can practice real-time flying and landing at different airports throughout the world.

The simulator is built on hydraulic legs which move it around to increase the realism.

Some computer games simulate realistic situations. Constructing towns, train driving, military exercises etc.

Chemical experiments may be simulated without actually performing them and production of gases may be studied using different concentrations of chemicals etc...

Biological studies of populations under different situations can be studied.

• Electronic circuit board simulators
• Kitchen design programs
• Architectural building - strengths and stresses
• Economic and financial modelling
• Traffic flow simulations
• Acoustic simulators for design of concert halls.
• Medical simulations
• ...and many many others...