In the original work they are a symbol of “hope”.
Shirou said that the modest wish kept within himself was wished not to the moon, but to the stars. Miyu hasn’t forgotten those words even once. The wish upon a star allowed by the vessel that should grant wishes. The glitters in the night sky are distant, and just being there his hands will not reach them. But his thoughts certainly reached something.
Until now, Miyu’s appearance has been drawn frequently looking up at the sky, absorbed by a deep feeling or another.

PRISMA material/petit: Encyclopedia of Prisma Illya MOVIE