A vacant property (really, any ownership of property, period) results in an opportunity cost externalized onto all other members of society. That opportunity cost is equal to the land's value. In the middle of nowhere, therefore, that opportunity cost might be low to the point of nonexistence, but in the middle of a city downtown, that opportunity cost can be and often is on the scale of tens/hundreds of thousands of dollars (or more!) per year. That is wealth that is long overdue to be repaid back to the rest of society.