Rollercoasters need to have a sizable power source behind them to get them to do what they do. The power required to launch a rollercoaster is immense but with a concentrated burst of Fluid Power, like that provided by a dana-sac.co.uk/fluid-power/ the rollercoaster can hit the high points and have the speed to get there.
Hydraulics and Pnuematics work in a similar way. At one end the fluid fills the piston that provides the power. When the pressure reaches the proper point it pushes the piston at great speed and power. This rockets the sled or car at such a rate of acceleration you get sucked into your seat as you feel the gravitational pull or G-Force.
This distribution and application of power have to be extremely well calculated. If there is not enough power the car will not achieve the maximum velocity to complete the whole circuit. Smaller hydraulic launchers give the car an extra push as it goes around. Too much power and the car could try to escape the rails. Both of these scenarios are very unlikely. One of the reasons that fluid power is used is it is very measurable and controllable. It is also more direct than an electrical or mechanical launch system.
All of these add up to a lot of fun. As the technology improves there is every chance that roller coasters will go higher, faster, further.