Brimstone wrote:

Relatively few roads have white lines along the side of the
carriageway. These are normally rural primary routes,
where the intention is to narrow the apparent width of the road,

Incorrect. The intention is to mark the edge of the road. The main
aim of this is to encourage motorists to keep away from the edge of
the road and reduce damaging to the bank

...by narrowing the apparent width of the road, which is what he
said.

How does marking the edge of the carriageway to keep people on it
equate to "narrowing the apparent width of the road"?

Because, in rural areas, the line is a small distance in from the edge of
the carriageway and the line itself, of course, has a width (often 150mm).
This has the effect of making the usable width of carriageway appear less
than if you ran your wheels along the blacktop/grass interface