Many computer programs allow users to accomplish tasks by dragging and dropping items on the screen, resizing windows, making adjustments with sliders, and creating artistic masterpieces using "virtual" art toolsall modes of interaction determined by the way in which the software has been programmed to deal with various events (mouse presses, mouse movements, keyboard input, and so on).
Responding to events lets you orchestrate your movies' interactivity by controlling when scripts are triggered. They provide a "when" to a script so that it executes only when something specific occurs.
The better you understand events, the more control you'll have over the user experience. By responding to events properly, you can create immersive environments the user will enjoy.