When I was writing a wishlist for Postcrossing, I added that I love to see traditional kitchens. Whenever I visit any castle, château, or mansion I make a point of visiting the kitchens so I thought there must be postcards of them out there, though not as many as I'd hoped. Later I added any traditional household interior.
As you can see, I'm collecting quite a few now. At least one is an advertising postcard but that's fine by me. They are a good record of social history.