Have you ever been to a housewarming party?
Was the house already warm when you arrived? I imagine so, unless you had some very inconsiderate hosts. Why call it a housewarming party then?
Well, it’s generally easier to warm a house now than it was in the past. In medieval times, the only way to heat a house (and cook) was with an open fire, so guests would bring firewood to quite literally warm the house. Apart from the obvious practical benefits of having a fire, this was also believed to ward off evil spirits, which is always useful.
Going by how expensive houses can be now in some parts of the world, I wouldn’t be surprised if housewarming parties become more literal again, and guests are asked to bring cash to pay for heating fuel!