I've already posted my speculation but the short version is maybe Marika's plan after she broke the ring/killed herself to try to get rid of the big bad thing was for Miquella to become the new Erdtree, so she made Empyrian twins (ie pure bloodline since she was both parents, that's what Empyrian seems to mean) and put all the rot in Malenia so that Miquella could be pure.
I think her main importance is to signpost how important Miquella is because she's waiting at the version of him growing from the tree, while meanwhile Mohg is keeping his real body undead elsewhere. He seems to think Miquella can come back and she might have been thinking the same thing, so you can guess at different conclusions from that.