It's only common with old houses built for renting, where the owner doesn't have to pay the energy bill, and where modernization as part of insulating the house

It's only common with old houses built for renting, where the owner doesn't have to pay the energy bill, and where modernization as part of insulating the house and building central heaters isn't feasible or economical eg. historic buildings. The devices are heating at night, and emit heat during the day to save energy costs. Not sure if this model makes much sense today. However, heating with electricity, as inefficient as it is, might actually grow compared to heating with gas or oil because of renewable energy subventions (similar story with e-mobility).