In some situations it is a much easier or better system than electronic motors or solenoids. A good example is when used in a situation with flamable substances or requiring very large forces.
A Solenoid Valve is normally required to interface Pneumatic systems to electronic systems.
It is possible to build fully automated systems with pneumatic components, which will function as long as air is supplied. Here is an example designed by Danny Staple at OrionRobots: Pneumatic Engine Design - including full LDraw System instructions.
A good alternative for larger scale applications is Hydraulic systems.
- http://en.wikipedia.org/wiki/Lego_pneumatics - Wikipedia Lego Pneumatics entry