دَخَلَ رَجُلٌ.
A man entered.
دَخَلَ الرَّجُلُ.
The identified man entered.
The indefinite expression need not identify the individual to the listener; the definite article can refer to a contextually established person. This does not mean that every definite noun must have been explicitly mentioned before. Shared perception and situational knowledge can also provide definiteness.
When a report’s purpose is to identify a specific participant, inappropriate indefiniteness may hide the needed information. Conversely, if only the occurrence matters, excessive identification may distract from the main predication.