Shutting down a computer is meant to cut down power consumption in a controlled way.
Shutdown schedulers enable users to schedule computers to automatically turn off according to pre-defined rules.
For example, the PC can be set to power off when its processor activity reaches a certain level, when there is no mouse and/or keyboard activity for a specific amount of time, when the dedicated counter gets to zero, when an application finishes its task and so on.
Usually, they display a warning before actually shutting down giving the users time to cancel the action, if they want it.
Also, some of these applications might also support restart and hibernate scheduling.