I have noticed that if you look at the actual death certificate-the "informant" is often a daughter or son. Ancestry then calls them "household" members. My research has shown the
"household" members may not be at the same address-but just the next of kin who is reporting to the coroner.
Same with marriage -the father and mother of the bride and groom are often listed -even if the bride and groom are no longer living with their parents.
Those are the best marriage records, pretty rare to find a mention of another family the further you go back. As others have pointed out they can really help you tie families together, at least loosely since usually they don't list relation.