Google's also saying this will lead to a simpler and more immersive user experience, where information from the suite of Google services can be combined to provide more relevant information while using Google branded products. In the video above, they give the example of how it can improve search results. Of course, it will also help target the right advertisements to each of us at the same time.
Google also wants to make it clear that they are not changing the basic elements of their privacy policies. They still won't sell your personal information, and they don't share it without your express permission "except in very limited circumstances like a valid court order." On the other hand, data about you can now be used across all services where it wasn't (couldn't?) before. This isn't neccessarily "evil," but it opens things up for a bit deeper discussion and review. In the end, Google is still going to be Google, and it sounds like the company's really only trying to simplify things for end users like us -- and at the same time making it easier for its own products to use what access you've already granted them.