What nuance does the `当` represent? In the following sentences:

 有一天他会当医生
 有一天我会当经理


what does the 当 mean? I think this means something like One day she would become a doctor. However, what kind of nuances does 当 include? Is it would be, would become, possible, or want to, or something others? How probable does the author think the noun after 当 occur?
当 is necessary here, it means work as; serve as; be.

有一天    他  会    当          医生
One day   he  will  work as     a doctor.


As you see, without 当, the sentence does not make sense.