It's to get people used to the idea without giving enough surface area to fight it socially. Same thing happened with the body scanners at airports in the US, now with facial recognition at airports, and a number of other privacy concerns. It's a well-worn and proven playbook.
At first it's a trial, experimental. Then it's standard, but optional and with incentives to do it. Then it's standard but you can opt out and there's lots of signs to tell you how. Slowly they make it more uncomfortable to opt out, and the signs disappear, and they berate you for trying it. Eventually it is no longer legal to opt out.
Any one step is hard to fight back against without looking like a loon to people who don't care about this kind of thing, and over the course of a few years you can take people gently through a process that, had it happened suddenly, would never have succeeded due to pushback.
At first it's a trial, experimental. Then it's standard, but optional and with incentives to do it. Then it's standard but you can opt out and there's lots of signs to tell you how. Slowly they make it more uncomfortable to opt out, and the signs disappear, and they berate you for trying it. Eventually it is no longer legal to opt out.
Any one step is hard to fight back against without looking like a loon to people who don't care about this kind of thing, and over the course of a few years you can take people gently through a process that, had it happened suddenly, would never have succeeded due to pushback.