I agree, it's not cost effective in purely financial terms.
Ignoring all the "awww, they're so cute with moma" stuff there are some distinct advantages for the fully free range and the locked in at night ranging model.
For keepers that free range, not necessarily on large properties and can keep males, broody hatching makes a lot of sense. Get this right and you can have eggs and broodies hatching. The broodies are not only hatching future generations, they are providing meat in the guise of the chickens one can't afford to keep for whatever reason.
This is my favourite model but for many killing what they've seen raised is too difficult.
One also does away with the weeks of chick care (incubated or bought) and integration problems and if the parents are healthy one can be fairly confident they will produce heathy offspring.
Good job I don't expect the allotment chickens to pay their way.