Probably something similar to what they were doing back in 2010 with Google Now. Skimming data from emails and texts etc in order to give you more pertinent information with Google Assistant's predecessor. The google now page of that time could tell me when my flight left, what gate it boarded at, if it was delayed, what airport entrance to use. It told me when my bills were due and how much. It tracked orders for me and told me when they were out for delivery or delivered. It would help me to pick a restaurant for a special occasion, direct me where to call or book for a reservation. I found it very useful and then privacy concerns basically tanked it.
We got a rebrand that did some of the same things with Google Assistant. And for a while that was really useful for a lot of the same things. But now that they've realised that data collection for this doesn't net them ever increasing profits there's a push for new better things. The new better apps and services don't really do what people want or need. They are specifically and ever increasingly meant to funnel more data to Google and more ads to consumers. There were a lot of potentially really good useful services that this style of scraping provided. But on the other hand, they didn't ask. They just took that info. And then saved face by sunsetting the product that people were gripping about.
If you use google services at all, google has a profile on you. Even limiting the spec of that profile by opting out of assistant and turning off a lot of their tracking doesn't necessarily help you maintain privacy. And google services includes their app store and phones.