I have come across Flash websites (the complicated and beautiful ones) that use multiple .swf movies. Why and how is that done?

You can imagine that as a module, Designers are create animations in different swf files and integrate as one to display in the website.

For example, When you refreshing a page frequently,the banner ads can change different swf's for every refreshing time.

I actually found the answer to my own question this time. You need multiple SWFs when making a Flash website and Action script helps make it happen. Thanks for trying to help, but it really has nothing to do with banner ads and everything to do with making a functional Flash website.