> I doubt there is any significant amount of housing that is just bought and left vacant. Why would you do that?

In high-demand areas property value often increases in value over time. You can literally “buy and hold” property, keeping it empty, doing nothing with it and expecting to sell it for a profit at some point in the future. It’s relatively safe and the rate of return can be significantly greater than inflation.

This ignores property taxes and costs of doing business of course. But there are also situations where you might want a reported tax loss on a profitable asset.