From what I get, making your own classes in AS3 simply makes the code or game or whatever run faster and avoid any errors because things are spelt out within the actions. I'm still learning but, most of what I understand so far says there not nescesary but usefull.
Yes, kinda, but AS3 is OO for a reason, plus it's a lot more efficient to write and debug code that has been properly written using OO programming methods. Before you think of coding any sizable game (anything over 200 lines), you should know what objects, classes, packages, interfaces, inheritance, encapsulation, and polymorphism are how to implement them.
I don't know about speed, but the idea behind OOP is that you can reuse programming / classes. Good OOP can use 30-60% more time to write, but it's easier to debug, and when written right, it can be reused often - which saves time on a new project.
Like Dank and PixelSmash said, by fully using OOP features, your code will be easyer to write, debug and reuse. But all these facilities have a price. To implement objects, their methods and atributes, you'll need tables in memory to store what methods and atributes belongs to what objects, and so on. That means that a non-OOP language, like C, is faster and needs less memory usage, but it is also more complicated to write complex code. That's why, even having to pay the price for the OOP, large companyes develop their softwares using OOP languages.
I finally got how classes work, it's like making a index of a book, so you don't have to read the whole book to find something, where the chapters would be functions you programmed yourself
And you can initialize variables inside the functions, so you can spare memory on that too
I don't get the logic, you spend more lines and have to make a call for an external resource, how can it be faster?
Following on from whats already been said, (resuable classes etc.)
In a massive application or game the source would be impossible to navigate if it was all written in one long file. With semantic class names it's much easier to work on a project (especially if you're working in a team).