The verb "ser" is used in these sentences for the following reasons:
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¿De dónde eres? - This sentence is asking about someone's origin or nationality, which is a characteristic aspect of identity. "Ser" is used to describe where someone is from.
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La chica es alta. - Here, "ser" is used to describe a permanent characteristic or quality of the girl, specifically her height. In Spanish, physical traits are generally expressed with "ser."
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¿Qué día es? - This sentence is inquiring about the specific day, which is a permanent aspect of time. The verb "ser" is used to indicate the current state of being regarding the day.
In summary, "ser" is used in these sentences to express identity, characteristics, and permanent states.