The current monarch, Queen Elizabeth II, became queen only because she had no brothers -- even a younger brother would have taken precedence. While British precedent was not as strict as Salic Law, where women could not inherit such titles at all, until now, male preference has been the rule. (The changes also remove the impediment about being married to a Catholic.)
This is a convenient time to change the rules, as it does not remove anyone from the top succession slots: Queen Elizabeth's oldest child is male (Prince Charles) and he has only sons, who do not yet have children of either sex. The first person it would affect would be the Princess Royal Anne, putting her ahead of her younger brothers, after Charles, William and Harry.
Imagine the effect on history of this happening in earlier times. Just consider Henry VIII and his multiple marriages, probably largely inspired by trying to get a male heir when he and his first wife Catherine of Aragon had only a living daughter, no sons. Mary I would have taken precedence over her younger brother, Edward VI, even if he had been born. And one step further: no Henry VIII? His sister Margaret Tudor was older and would have taken precedence once the eldest sibling, Arthur, died. Queen Victoria's firstborn was a daughter. Just a few among many times that history might have changed had this change come a few hundred years earlier.
Read more about women who've ruled as queens and empresses, or even wielded power as significant regents:
- Empress Matilda (Maud) - whose succession was in doubt primarily because she was a woman
- Medieval Queens, Empresses, and Other Women Rulers
- Women Rulers of the Early Modern Period (1600-1750)
- Women Rulers of the Eighteenth Century
- Women Rulers of the Nineteenth Century