>why does Google and their advertisers need to know about it I would ask
Google is pretty clear about this. The only reason they track you is for advertising, and there isn't any evidence of them using the info for anything else. In fact there is a lot of evidence pointing the other way, such as their insistence on encryption data flowing between their datacenters.
This is Google we are talking about, not Kazakhstan, China or Russia.
Google could eventually use this information to determine your eligibility for a home loan. They have already dipped their toes in this area [1]. With all this data, we have to ensure that it is used fairly (or not at all). There is enough concern about digital redlining that a 2014 report to the white house reports on this [2]. As we know machine learning is quite capable inferring sensitive attributes [3].
This inference doesn't even need to be intentional, machine learning is capable of accidentally picking up on latent variables. Even if your neighborhood (the original redlining) isn't a feature in the original variable, it could be inferred from the other variables.
TL;DR: Your surfing behavior could be used to deny you a home loan one day.
Lots of speculation about what they might do. You could also say that the US government could use all of your data to spy on people who criticise the government, so they shouldn't have any of that data either.
Their other purpose is "dont be evil". There may be some debate about that at times, but they certainly aren't going to screw their customers. They know that their customer base would evaporate pretty quickly if they tried to screw them.
Google is pretty clear about this. The only reason they track you is for advertising, and there isn't any evidence of them using the info for anything else. In fact there is a lot of evidence pointing the other way, such as their insistence on encryption data flowing between their datacenters.
This is Google we are talking about, not Kazakhstan, China or Russia.