Siri tries very hard to be a true "agent"; that is, the makers of the technology strived to make Siri something that would perform tasks on your behalf and at your command.

Clippy was also intended to be an "Agent", but it failed spectacularly at it because:

1. It tried to help you when you didn't want help, like an annoying intern who won't leave you alone.

2. When you actually wanted it to help you, it was invariably unable to perform the tasks you wanted.