They keep the power, water and HVAC system operating to keep the building from getting damaged by neglect, fire or frozen pipes and such. Homeless could be a part of the equation, but I would think theft of wires, copper pipe and such is a greater potential damage. Not to mention someone may sue them if they entered the building and got hurt. It could also have a new owner and it can take a year or more to produce plans and get them approved by the city before changing the use. Always good to keep your eyes open and notice what's going on around you.