In short, Google Now is robust enough to act as a true personal assistant. Yes, your iPhone can do much of this out of the box, but you have to use separate apps like Calendar or Reminders. Google Now puts all that servicey stuff into one all-encompassing app.
And Google Now is much better than Siri because it pulls in data from the Google services you already use like Gmail, Google search, and Google Calendar. For example, if Google Now sees that Amazon sent you a package shipment notification to your Gmail inbox, it'll automatically pull in the tracking information. It can tell you when to leave for work so you make it on time, taking traffic into account. It can tell you sports scores for the teams you search for the most.
And so on.
In short, you don't have to do any work. Google Now gives you what you need without you having to ask. Siri, on the other hand, still requires you to ask for help. And even then, Siri doesn't have the same useful information Google does.
But here's some good news for Apple fans: Apple is working to make Siri more Google Now-like. It recently acquired a company called Cue, which much like Google Now, pulled in data from commonly used services like Gmail to bring you the information you need without you having to ask.
Based on that, it's not a stretch to say that Apple sees the value in mining your personal data to create a more useful Siri.
For now though, Google's new update to the search app for iPhone simply smokes Siri.
Here's a video that'll give you an even better idea of how Google Now works on the iPhone: