WTF are you talking about?
Her character doesn't add anything to where the storyline is
NOW?
Cotdamn.. you have not been paying attention.

Lemme help you out...
The very FIRST SCENE (of Season 5 Episode 1) started off with a ‘young Cersei’ visiting a witch in the forest who foretold her future.
The Witch told her: "You will be Queen for a time... then comes another. Younger. More beautiful.... To cast you down and take all you hold dear."
Cersei thinks the prophecy is about Margery and the Tyrell family.
But more than likely that witch was really talking about Khaleesi.
So pretty much everything that is happening RIGHT NOW is because of Cersei trying to PREVENT this prophecy from becoming a reality.
No need to give you any lectures, chief.

You probably wouldn't be able to understand them anyway.
Honestly, I think this show might be too much for you to follow.
Just saying.