Geralt's role as "the witcher" separates him from normal society and forces him to be a rootless outsider so he can wander around giving the audience an outsiders perspective on the violences and prejudices of the world around him.

Putting Ciri in a similar role is a literary conceit. If they want her to wander around meeting weird people, helping them with their problems, and sleeping with them she needs a reason why she's wandering the countryside instead of doing important elf princess of magical girl destiny stuff.