That is, imagine you have a big city. You can add capacity for 1m extra people to travel to the city centre, where there's lots of congestion. Or you find ways to induce demand around the other limits of town, even town current demand is low there.
Odds are you'll pick the first, because it's "obvious" and doesn't require much thinking to see it'd help. But we really ought to look at cost-benefit of the second option too, because repeatedly inducing demand in the centre keeps driving up the incremental cost of further improvements, along plenty of other undesirable second order effects.
It's obvious at the supermarket: what goes faster, a single cashier processing four short lanes of 10 people with round robin, or two cashiers processing a single lane with 40 people?
Is the city center able to process 1m extra people? If not, it doesn't matter how many lanes you build.
E.g. London's "Crossrail" / Elizabeth line cost $24 billion. Granted, it also allows some people to go through London faster, but I can't help to wonder what that money could've done if applied to attract businesses out of the centre instead. E.g. upgrading links between towns on the outskirts, upgrading town centres, and generally try to make it more attractive for businesses to be located further out.
Given the extraordinary costs it takes to do large infrastructure projects in London, I'd be very surprised if you couldn't get a higher return on investment that way, or by investing similar sums elsewhere in the UK entirely.
It does not reduce congestion, but it does now serve more people at this same current congestion level. And those people have come from somewhere. Sometimes from public transport, which isn't really good, but sometimes from some backwater road.