Siri isn’t a search engine, although it shares some of the same goals. Siri is, in essence, a new user interface, whose objective is to perform tasks, not merely deliver results.
Thus Siri, even in its infancy, performs a wider set of context-driven activities that range from setting an alarm, to finding the nearest restaurant, to translating a user’s question into a request sent to a search engine.
Siri is glue. It aggregates a diverse set of services and resources into a single voice-driven interface that doesn’t require the user to know exactly which site, service, or app will best complete the task or deliver the most relevant results. Ideally, Siri will know which of its underlying components is best suited under the circumstances.
You tell Siri what you want, not how to do it.