I think it was to show that she is losing her faith. In the after the episode inside the episode thing, D&D said she was centuries old. I mean think about it, she thought Stannis was the lord of light, and he died. She burned a kid for that. Then she thought she saw jon at winterfell but he's dead. In waaaay past, when she was talking to Selyse she had potions and said that she can look however she wanted to get what she wanted. I think the point of her not worrying about that and going back to her old age was her losing all faith of what she was told. not that the lord of light doesn't exist but that she was told wrong. If she's centuries old, that's a huge slap in the face for hundreds of years of faith.
The same thing happened to Thoros of Myr, or so he said, he lost his faith and welp, we know how that turned out
So I think this is important for her to hit a low spot before she brings Jon back *fingers crossed* or at least that's what I hope.