Still trying to figure out why she has them. Currently reading Medea, but no help there yet.
Most likely it is simply to boost Caster's attractiveness, but the designs for Fate characters usually aren't done out of nowhere.
It's either that orrrr....
To make her look more the outcast and puts focus to her being shunned in her past since she hates being called "witch" and all that.
But it's probably to make her more kawaaiiiiiii.
Which is fine.
Saber was originally a dude but they changed him to a chick for obvious reasons (according to Lotion anyway.)
But they made up for it with great characterisation and writing. Overall it didn't harm the character in any way at all... hell, it made her even better in the end.