Right, but I still don't understand what underutilization problem is solved by property tax that isn't already solved by the fact that you could sell your underutilized land and receive money — unless your goal is to increase utilization beyond what's efficient.

People aren't rational actors. This has been demonstrated in many econ experiments. Tax is a pain that is reoccurring. The government even reassess the value of your land periodically. Many people don't do this and may not know their land is underutilized. It is essentially a little nudge every year.

In this case, maximum utilization means maximum economic output, which is good for GDP and good for the country. It may not be the best for the local society, though. That part is hard to quantify and controversial.

Why are there abandoned buildings, parking lots and urban farms in downtown and near downtown neighborhoods in most US cities? I lived in a converted former warehouse that sat empty for decades because the owner had a dream that a factory would eventually rent the space again. It took the owner's death and his son inheriting​ the property for it to be put to "efficient use" again.

The Georgist argument is that the problem is property taxes aren't based solely on land value, and this creates excessive deadweight loss from taxation. https://en.wikipedia.org/wiki/Georgism

Under a system of pure land value taxes, an empty lot downtown would be taxed at the same rate as the highrise next to it, and taxed much higher than an empty lot in the outskirts—a huge incentive to put that empty lot to more productive use.

The proliferation of parking lots is a regulatory/zoning issue.

Worthless property is worthless and the carrying cost is low.

If you have the capital, sitting on low value property can make you a lot of money.

Also, if you are a real estate investor, you can "launder" your profits to avoid taxes. That's why old shopping plazas will degrade until there is only a kung-fu place and a dollar store. It literally has a value sopping up real profits from thriving holdings with paper losses.

Why do so many places in London owned by foreigners that never live there?

As old Marx put it, a commodity has two values. A use value, and a exchange value.

In the case of land, usage would mean building on it, farming it, or some such activity.

But even if nothing like that happens, the exchange value remains. Meaning that someone out there is willing to trade you for it.

It isn't really.

In a discussion about problems with urban property in the US, an HN thread would decry property tax and wax on about land value tax.