1. "for good measure" : The king needed an "extra" heir, JUST IN CASE the first one died or was in some way disqualified from becoming king, then the king would still have another in line to take the throne. Some other examples:
If I was told by me professor to write an essay that was at least 5 pages long, I might write one 6 or 7 pages long "just for good measure," just to make sure I fulfilled the requirements.
Or, if you were cooking a meal for your mother, and she enjoys it very salty. After making it salty enough, you might put in a couple extra dashes of salt "just for good measure," just to make sure it is really salty enough.