فمقام كل من التنكير والإطلاق والتقديم والذكر يباين مقام خلافه
The stations of indefiniteness, unrestricted wording, fronting and explicit mention each differ from their contrary stations.
Illustrative contrast:
جَاءَ رَجُلٌ — A man came; the referent is not yet identifiable.
جَاءَ الرَّجُلُ — The man came; the referent can be identified from shared knowledge or context.
An indefinite noun may suit a first introduction; a definite noun may suit an identifiable person. These are explanatory examples, not an exhaustive statement of every rhetorical function of definiteness or indefiniteness.