I don't know anyone that has an empty room in their house, or one that no-one enters for weeks at a time, etc. Maybe the odd country estate is like that.
To a communist, the concept of having a guest room, office, storage room etc might feel like "underutilization".
If we apply that more generally, then the park outside my house is underutilized because the maximum capacity are not sunbathing in it at all times. I think that's a pretty silly use of language.