If you own a large amount of land than the savings add up. Especially if you live 250 years ago (or you want to match the walls from then) when bricks were not produced and delivered in massive industrial processes and large estates were more common.

Even in the modern era the cost is still relevant. Bricks are still pretty expensive.

If I have 100 acres (square), I need ~2.5 km of wall, at ~150,000 bricks for a 1m wall single brick-width wall (deter animals, mark property).

At the online prices I'm seeing ($0.65), that's ~$100,000. If I have to make it all double width, suddenly its $200,000. $100,000 delta is still pretty relevant for a modern small scale farmer.

The difference is people enclosing 100 acres today are either wealthy estates or using non-brick fences for farmers.

Historically, lots of countries had laws saying you had to enclose your land. If you didn't, then you might lose it.

In the days before wire, brick walls were a cheap longlasting enclosure method, especially if wood or stones weren't easily available.

wire dates to ancient sumeria i think; it was used for money before coins (because it's easy to measure out any desired amount of gold or silver)

It is if you're selling sheep milk and you don't want to lose your flock.

I think that’s typically a job for fences, right?

This sort of wall is, I think, just an aesthetic way of marking a property line/get some privacy.

Drive through rural northern England and you will see vast numbers of sheep moving through pastures that are bordered by old dry-stone walls. The roads will even have equestrian gates alongside them when they have stock grids to prevent the sheep from using the road.

It's all about adapting to local materials. The same technique was used by early settlers in New England (think about the ending of The Shawshank Redemption) because they had to get the stones out of the ground in order to plow and harvest - rather than just make a pile, they used the stones to build walls separating fields.

Depends on how long you intend to keep livestock and what materials you have access to. Well built walls can last a lot longer than well built fences; but fences may be less costly initially. But it might also depend on how crafty/destructive your livestock is.

I have a few acres of land and annoying neighbours. Stuff like this is relevant (though in the end I just went with hedging, which is cheaper and good enough for privacy)

It's relative. You might be "tight on money for building a wall" so you save money by building a wavy wall.

Pre industrialism, almost everyone was dirt poor by current standards.

They were also dirt poor by current standards after industrialism started. It was well into the 19th century that laws like the Education Act of 1870 and the Trade Union Act of 1871 started distributing power to the common people of Britain outside of the traditional quasi-feudal system.