An article about Old Land Deeds, from The Pennsylvania Magazine of History and Biography, Volume 41:
Joint deeds of this sort are invaluable to the genealogist and should always be looked for. They of necessity give the full name and exact relationship of every living heir and also of every husband or wife of an heir. The place of residence of each heir is usually stated or it can be inferred from the notary's certificate or witnesses to the signatures. All of this makes a family history about as complete as could be desired.
Even ordinary deeds are worth looking up, for real estate transactions between relatives were much more frequent then than they are now. The witnesses were often relatives too. At any rate it always shows whether or not the grantor had a wife living at the time, for she would have to sign it too.