It's "cost neutral" in the sense that the company still pays the same $X to run the office regardless of how many people are in the office. But if it costs $1000/day to heat your office in the winter and only 50% of your employees are working in the office any given day, you're wasting $500 worth of heating that day.
Looking at it from an overhead perspective, let's say I have 1000 employees and my heat costs $1000/day. When all my employees are in, it costs $1/employee/day to heat my office. If only half my employees are in, it costs me $2/employee/day. My overhead per employee just doubled.