I have no idea of the history or structure of the building but the current vogue, especially in the cheaply refurbished for rent world, is to block up as many openings as possible so as to prevent drafts and heat loss. Now if the house is old, say Victorian, every room would have had a fireplace and since coal was cheap, a fire burning with a skivvy running up and down stairs all day with buckets of coal. Those fires or even just a kitchen range and living room fire would have sucked massive volumes of damp air up the flue, bringing in fresh air from outside while warming the structure of the house. In an old, poorly-insulated building if you prevent that exchange of air the damp air inside will drop its moisture in the form of condensation on cold surfaces, especially in areas with little air movement behind beds and cupboards.