I know your question was rhetorical, but let's give it fair consideration. I suspect what they'll learn is whether they have the capability to analyze the movements of their entire population efficiently and if not, what it would take to do so.

Armed with that knowledge, they can prepare for the eventuality where the one-off exercise gets turned on permanently.

When people go where they go, and where they go next. This can help to figure out which interventions are even worth trying to improve things for public transport. I can imagine this would be very useful and relatively cheap to do.

Actually I have worked on projects for major UK rail stations along these lines - how many people on this platform then go to this platform, and should we build a bridge / open up a barrier.

I can see that as useful - it is targeted and pretty simple to do (although the anti-tracking mechanisms in wifi now makes it harder)

but ... I struggle with the value of such data collected on such a scale. Cell Tower radius is on the order of kilometres - the interesting stuff to do inside cities cannot be discovered on that scale - you need metres or better.

This feels like a mass public holiday migration watch. And you learn that ... people live in different cities to their parents?

Cells have both a maximum range and a _maximum population_. The multiplexing that enables many devices to use the same spectrum to communicate with a tower is limited, so you can't just put up one tower in Manhattan and you're done, that'd fill up with users immediately, you need to add lots of towers in areas with many simultaneous users or the service is lousy. As a bonus the reduced transmission distance means handsets (automatically) reduce radio power, which means improved battery life.

As a result cells are naturally much smaller in populated areas where you're most likely to want detailed information. In a city centre the cells may be only a hundred meters across. True, up a mountain the cellular network may have no idea where on the mountain you are, but this isn't a Safety of Life application, it's a survey.