Google has an Analytics product so that website owners can see how much money they make from AdWords, thus encouraging them to spend more money on AdWords. GA is a strategic complement to Google's advertising services, and that is how they derive value from it.

Google does not dip into GA data for their own uses. Post-GDPR this is extra-explicit because they take pains to clarify how much they are not a Controller and only a Processor, but this has always been in their TOS. Frankly, most people's GA implementations are dumpster fires and trying to make use of that data would be more cost and less benefit than punching themselves in the dick.

Google does look at GA data on occasion, but only for the purposes of debugging and for ensuring compliance with the TOS (e.g. they will flag accounts that contain PII).