By reducing the days 2nd class is added to the deliveries. The number of houses the. Postman has to stop at is hugely reduced on those days. Meaning the routes can be finished faster. R more routes by less people.
At a guess I'd say in an average street those days would have about 25% less stops/driveways to walk down etc.
It will have little or no effect in large blocks of flats where all postboxes are in the same room. More common in cities. But for suburban and rural deliveries the effect will be huge. Even in terraced streets with no front garden. It would be a small but notable change in time taken.
PS. Not saying it's a good thing. Just pointing out why a corporate money hustler would want to do it.
This is about being able to charge more for Saturday delivery like most other "premium" delivery companies as they sure as shit wont be cutting the hours that they expect the Posties to work.
Because second class has a decent service level, even now, its possible to time your parcel delivery for a Saturday reasonably accurately for a low(er) fee. Drop second class then push up the prices for 1st class parcels.
Then when they can, with a more friendly Ofcom, they will stop first class on a Saturday and switch to a proper Saturday delivery tariff.