Callbacks make it easy for you to respond to such things as incoming MIDI messages, plugin parameter changes, timers and so on. While many languages today simply define regular functions and then explicitly bind them to system events, GP Script uses explicit syntax to distinguish callbacks from regular functions, making it easy for you to differentiate between functions that you call vs "functions" that the system calls and to which you respond.
More in this chapter:
Rackspace callbacks
MIDI Events
Widget events
Plugin events
Scriptlet events
Generator events
OSC callback
Playhead events
Global state
Global events