The idea is that the country is never without a monarch. The instant one dies, the next in succession takes his place.
Unless you're Richard, Duke of Gloucester, and take possession of the new young king, son of your brother Edward IV, while he's on his way to London to prepare for his Coronation. You murder his uncle who is escorting him, and then you spirit him, and his only brother away to The Tower of London, after which you declare his brother's marriage to their mother (the Queen) was never legal, and all children born of that relationship are declared illegitimate, and not eligible for the throne...then you claim the throne for yourself, because your oldest brother has already been executed as a traitor, and his children removed from the line of succession.