Especially in constrained areas like Barcelona (or urban areas in the U.S.), property ownership is a monopoly, so it has market repercussions. Buying up units and then refusing to rent them out (or have anyone occupy them) can be viewed as interfering with the local market. These are micro markets that are price sensitive to interference.

If you are a landlord, you need to rent the place out at the current market price. If you do not want to do that, you can sell the property and get out of the landlord business. Just squatting on it and leaving it empty/boarded up distorts the local property market and should be heavily taxed to discourage absentee landlords from interfering with local markets.