this alone can be used to correlate a huge number of things since very often you have one household with a whole assortment of people and tablets, phones, laptops, xboxes, playstations, smart speakers etc all behind one IP address and NAT.
just having an entity like google know that your ip address is coming from a /24 in a /19 sized IP block that's known to be a comcast dhcp pool in a particular city/metro area is a huge amount of info.
other things happen as well. you have other people in your household who installed the facebook app on their phone and left geolocation/GPS permissions set to on? now your IP address has been correlated with a very specific latitude/longitude and the usage patterns of your (spouse, roommate, child, etc) who is using that app.
> collected for specified, explicit and legitimate purposes
If a malicious person had access to your account you want to know for how long they had that access for example.
> Store it because nobody knows what to do with it?
I think the technique, itself, is decades old, but has been vastly improved, using AI and algorithms.
It can be used to match things like anonymous rants, to individuals.
But that is exactly how this works. From what I understand, the “enhanced” version of the technique is scarily accurate, able to match, using just a couple of sentences.
Books written about the NSA, GCHQ, CSE etc talk about them using things like timestamp logs/traffic analysis/time of day analyzing commercial and government telecom links going back to the 1960s, so in the modern era even if your crypto is absolutely unbreakable, there's a huge amount of analysis and correlation that can be done based on timing analysis.
Then you combine your timing analysis with things like correlating geolocation of blocks of IP addresses, netflow and traffic analysis, metadata obtained from other adjacent/nearby users on same ISPs at either end, a whole fire hose of other data that's still useful even if the crypto is solid.
> let's say Google has already identified your Reddit account. You open a Reddit discussion, and deep in the discussion it links to a Youtube video, and you open it in your browser. Now even if you weren't logged into Youtube, Google could guess that it's you based on the timing
Not just timing but also cookies, client device/browser fingerprinting, IP address/what ISP you're on, and the usage patterns and logged in activity (and app-collected telemetry data on android and ios devices) of all the other persons in your household and neighbors.