Did she have a vision of it’s future need?
Or is it like how those who have faced starvation compulsively hoard food?
“May it be a light to you in dark places, when all other lights go out.” Says someone who lived through the darkening of Valinor, when light far stronger than the sun and moon went out, and took all the safety and sanity with it.
Three ages of the world later she is moved to capture the echo of the Silmaril that sails the void in a glass vial. Despite all the horror that her family capturing light in artifacts has historically caused.
Just in case.
And then faced with the days growing darker, she faces the same choice her uncle did with his creations: hold on to paranoia, and keep it close. Or give it away, that it might go where it can do the most good.
And she chooses to let it go.
Exile to middle earth wasn’t a problem, until her daughter needed to go to Valinor to heal. Now, she needs a way to get to Valinor when the Valar have not forgiven her.
Because she WILL see her daughter again.
She only knows of one thing that has gotten a ship to Valinor when it was fenced from the Noldor- A Silmaril, carried by Earendil. And so she sets about capturing the light of Earendil, that one day she might trade it for entrance and keep her pride.
But, turns out, the Valar sent a different test.