House Contractor is a shade who responsible for building and upgrading stuffs that can aid you escape from the underworld. However, some of the upgrades are just to progress the storyline or for decoration only.

Usually, when you see a word "Decor" in the upgrade descriptions, it means that item's purpose is for decoration only and you should NOT upgrade it and prioritize other items that are more useful that can give you stats or enhancements. Only if you don't have an option then you can buy these cosmetics.