In short: Google Assistant has picked up the ability to go through the entirety of the purchase process by voice alone, including logging a user in for security purposes, confirming a stored payment method, and making a final confirmation of the order. On top of that, developers can now offer, and users can now buy digital goods through Assistant. As a bonus, the Google Login API that's accessible to developers on mobile platforms is now accessible in Assistant experiences, meaning that a developer can simply pull account information from a user's Google account if they're logged on, in order to populate account information for that experience, rather than having to either create a unique login system or use OAuth to access Google accounts.
Background: Buying physical goods over Google Assistant has been possible for some time now, and is even one of the main uses of smart speakers across platforms. Although the process has not always been the most seamless which is where this update comes in as it's designed to make the buying process more fluid from start to finish. Previously, when users wanted to pull their account details from an Android app or other service tied to their Google login, developers had no choice but to create a specific login for the Assistant experience then put in a roundabout link to the user's Google login and their login for that app on other platforms, or they had to use the OAuth service.
Impact: Not only will developers have a much easier time creating and maintaining user logins now, and be able to expand their suite of available services via Google Assistant, but things are going to get much easier for users, as well. Users will find that they're now able to log into Google by voice alone, and that login can be used across all sorts of services. Thanks to that, any purchases they make via Assistant can apply across the Assistant experience and the main app. Subscription services and one-time payments are both available as of now, opening the door for Assistant experiences with an upfront cost in the future. This opportunity to make more money will doubtlessly entire more developers to head to Google's ecosystem, helping in the battle against the ever-popular Amazon Alexa.