Well two big ones are dialing while driving, esp. from a big contact list, along with sending text messages and email while walking or driving. You can voice dictate emails, messages, etc. and don't even need to look at the phone when hands-free, or have it dial without having to divert your attention from your route.
Another is locations. I don't use iPhone for GPS navigation, but if you quickly need to find something nearby, you can ask and it is pretty darn accurate, and superior to most car navigation systems.
If you are sitting down or at a desk, no, I don't use it. But if you are on the move and can't type, or need your visual attention directed elsewhere, that's when Siri is very useful. You can operate the phone without looking at it.
That's how I use it at least.
I also imagine people with poor hand-eye coordination use it for typing. The transcription is very accurate, the best I've seen.