As a replacement for ActionScript: contains several powerful language features, including ‘type inference’, and can compile the same code to flash 6,7,8 or 9. No need to switch syntax when going between versions.
As a replacement for PHP(Perl/Python/Ruby/etc): Use the same language on the client and the server. Libraries for file/database/image/etc manipulation. Compiled to bytecode, thus can run much faster than the interpreted languages (some benchmarks show it 20 times faster than PHP).