Play Billing 2.0 added the ability to purchase in-app items from outside of the application.
This ACK confirms that the application properly processed the notification and delivered the item to the user. Otherwise, Play assumes that the purchase was lost in the ether sometime before fulfillment (e.g. bug, the app crashed, etc.).
From the post, it sounds like the users receive the purchased item immediately but the app developer needs to confirm that the charge went through properly. Otherwise the funds are returned but the user keeps the purchased item.
Without acknowledging the purchase the money was refunded back to the people who purchased it. And the best thing was the dark theme would be activated even if the user got the money refunded.
This ACK confirms that the application properly processed the notification and delivered the item to the user. Otherwise, Play assumes that the purchase was lost in the ether sometime before fulfillment (e.g. bug, the app crashed, etc.).