Games are not movies. Movies are linear with no interaction. Games may have a linear story, though often that is not the case, but they have interaction. Interaction means that the application will behave differently based on what the user is doing. The best example of this would be the user clicking on a button. The Animate movie has no idea when the user is going to click on the button. Likewise, if there is more than one button, the animate app is going to need to know what to do based on what button is pressed. To deal with this, we need a scripting language.
Create.js is actually a collection of four different libraries. These libraries can be used individually, but tend to be grouped together. As Animate generates a lot of the code for you, the bulk of the code that will need to be written is game logic code with direct manipulation of the Create.js very rare and often only to adjust already existing objects.
Tween.js handles the animation aspects of the program. As explained earlier, tweening is simply changing an aspect of an object over time so it is possible to use Tween.js for non-animation related aspects of a program.
Preload.js is an asset preloader. This simply means that you give it a list of assets that you want your program to use and it will load them in. The preloader generates events to let your program know when it has loaded assets so you can even create a fancy loading screen if you wish.