Until now, every single Google property acted like a separate company. Due to the way we grew, through various acquisitions and the fierce independence of each division within Google, each product sort of veered off in its own direction. That was dizzying. But Google+ is Google itself. We're extending it across all that we do — search, ads, Chrome, Android, Maps, YouTube — so that each of those services contributes to our understanding of who you are.
Redesigning all Google services to match the Google+ interface is not just about consistency, it's also a way to show that they're part of the same super-service, an encompassing layer that makes them work together.
Google+ made Android's camera app integrate with Picasa Web Albums, Google Talk integrate with YouTube and Google Docs. It's likely that Google+ will make Google services more useful by combining their strengths. That's probably the reason why you'll use Google+ even when you're not going it to Google+ and you'll end up joining Google+ even if you don't like social networks.