Google is today opening Assistant to developers in a way that will allow them to integrate in-app purchases and subscription plans with their products. Furthermore, Google Sign-In is now also available in Assistant, allowing users to seamlessly log into their Google account and make purchases using only their voice. This means you'll be able to buy all sorts of digital goods without having to enter your password or use touch.
Some notable apps already taking advantage of the new functionality include Starbucks — which lets users their Starbucks Rewards through Google Assistant and earn stars when they make purchases — and Meditation app Headspace, which lets users subscribe to their plans and upgrade their experience while through Google Assistant.
These changes come alongside a major Google Assistant redesign on phones, that is focused on delivering visual content in more engaging and convenient ways.
source: Google Developers Blog