Why? Because apparently they need some more incentive to keep units occupied. Also, even though a property might be vacant, there’s still imputed rental income there. Its owner is just receiving it in the form of enjoying the unit for himself instead of receiving an actual rent check from a tenant. That imputed rent ought to be taxed like any other income.
Why? I bought a house and lived in it for 15 years, then my wife and I bought a new house and are renting out our old house with the aim to use the income from it to help with early retirement.