The article makes the point that people do not build fences for no reason. It costs time, effort and resources, not to mention it's a lot of work and people are lazy. There's always a reason for the fence.
>or the reason is completely lost.
This may be true. A fence in the middle of a wood may have been put there 90 years ago because of old property demarcations, or an effort to keep the dread bearded grindlesnatch from attacking the village. The property is now owned by one person, and climate change killed the bearded grindlesnatch, so the fence isn't needed anymore. But the point is to find out why it was there before you tear it down, and if you can't find the reason, you should be extra careful about yanking it down. Perhaps now the fence harbors a mini ecosystem of berry bushes that has increased the potential environment for wild game.
Progressives tend to be very dismissive of the Chesterton's Fence analogy, because they think change is an unalloyed good. There are a lot of social mores or codes that seem unnecessary, outdated or even bad, but they don't want to look at the reasoning behind them. Second order effects are easy to dismiss as speculation, so they usually are dismissed; but any change, radical or otherwise, will have second order effects, so careful thought must go into them.