Taxing vacancies breaks the rent logjam caused by financing agreements that allow you to tack missing rent onto the end but require recapitalization for lower rent.

Individuals and small landlords raise or lower rents in response to market conditions as they prioritize cash flow--it's very difficult to make up for lost rent. It's the private equity financed stuff that is artifically keeping rents too high as they have enough cash to ride across an empty property almost indefinitely.

Vacancy taxes stop the idiocy and force the private equity financed stuff to be market responsive as well.