It would make the cost higher. When you increase the cost of something, fewer people do it.

Real-estate as ~The Ultimate Investment~ sitting in hedge fund portfolios with the motive of maximum financial extradition of the local population could easily eat the cost of empty units at the gain of forcibly starving out available living space in urban areas.

A tax crafted in such a way to disincentive hording and speculation would push empty homes and apartments into actual use as living spaces instead of financial siphons.

I still don't get it - how are they making money if they don't rent or sell those housing units?

I get that they speculate on rising prices. But that is still a speculation, so it can go wrong.

And by driving away people, they are probably devaluing their property.

The purpose is to put to use housing units. Not speculative financial devices.

Hording real estate in urban areas inflates prices via intentional limitation on supply.

I get that, and it's an age old accusation against speculation. But it's really not so simple, because limiting the supply also has costs and it might ultimately fail (it usually does).

Do you have any numbers that show the speculation is really what is driving the prices? I doubt that some vacant flat here or there has a significant impact on prices.

If a street was completely vacant, prices would probably go down because it would be unattractive to live there.