People see license plates all the time, but they don't normally stand there logging them for hours each day. There's lots of data where this sort of distinction matters.
For example, I'm a type 2 Diabetic. I voluntarily disclose this, and that I use a blood glucose meter and take Metformin for this condition. Now suppose somebody, perhaps working for my insurer, wants to check such data as what dates I refill my prescription, and what times of day I test, how regularly, and so on. There's several potential problems here. First, if my insurer wants to claim that I have been getting refills irregularly, it's in their interest if there's a law keeping me from stockpiling my medications, because that might be an alternate explanation for why I might go more than 1 month between refilling a 30 day supply.. Sure enough, there are an increasing number of drugs which don't have any known abuse potential, but that the prescriptions can only be filled for 1 month at a time, by law. The Insurers are not just interested in writing their rules so they don't pay out for multiple months at a time, but getting states to actually pass laws, which suggests to some of us that they really are trying to track such data in hopes of denying more claims. Then the test strips and lancets themselves are available in at least most states without a prescription. Again, there's no real abuse potential there, but again, there have been insurance lobbyists advocating making these items prescription only.
This sort of data is routinely observed by at least one other person (the clerk) any time I buy these medications. There are other people, such as my doctor and the pharmacy staff who may sometimes ask me if I'm testing regularly or remind me about proper use of the test kit and meds. But the insurer isn't just some party that presumably has my interests at heart in the general sense, they are an entity which might want to deny a claim if my disease gets worse, by claiming that it's my own fault for not following all the instructions adequately. The insurers are also people who have already lobbied for laws which would make my life a little more difficult. (For example, if I have to get a prescription for test strips and lancets, then I have to contact my doctor if my meter breaks and tell him what type I buy as a replacement before I can start using it.). So, the individual data is not kept particularly private. I'd let my doctor or pharmacist see the meter and page through the log stored on its SD card pretty much on request, and if I have the insurer pay for my meds, they presumably can see what dates I've filled the prescriptions and could track them easily. Yet, there's still problems with the access they already have, and piecing together that data gives them the power to do some things that can be a real pain.Piecing together more data is likely to open up new areas for abuse.