What is SWC?
SWC is a file containing ActionScript source codes in a compiled status. The good thing about a SWC file is that it will help your projects compile faster because your compiler (flash CS4 for example) doesn’t have to compile everything from scratch any more.
In simple words, you use SWC files to access class packages instead of accessing the class .as files manually. This is how flash and flex source codes are orgenized also. There might be other good things about SWC also but that’s what I know about, feel free to let me know if there’s anything else 
It’s very hard to explain the features of this class! because it seems like a very basic small piece of class! but beleive me, it’s more than a simple background