If you buy a new car, you have to tax it even if it already is taxed by the previous owner.
The reason you have to tax the car when you buy it is to ensure that it is insured and has met the requirements for the MOT. They won't give you new tax until it passes those two tests. Under the old system you could happily drive around with no insurance or MOT. The ANPR camera would have to check three things, now they only have to check for tax. QFS if you think about it.