Think of the English "while", it can connect two contrasting sentences. Not in the sense of "during" but rather in the sense of "and" (I am a student while he is a salesman= Я студент, а він продавець)

You cannot use it when the statements are the same, sounds strange (I am not a student while he is a student = Я студент, а він студент ~~> Я студент, і він студент.

i means the correspondence of the two actions ( я їм, і він їсть). Reading and listening are not alike, so we connect the two with а (meaning I am listening to music and he (he is not listening to music) is reading magazines