The verb "ser" is used in these sentences for the following reasons:
-
¿De dónde eres? - This sentence uses "ser" to express origin or nationality. "Ser" is used when talking about where someone is from.
-
La chica es alta. - Here, "ser" is used to describe a characteristic or inherent quality of the girl (the fact that she is tall).
-
¿Qué día es? - In this case, "ser" is used to indicate the identity or nature of something, specifically to ask about the day.
In summary, "ser" is used for origin, characteristics, and identity in these sentences.