Annexation identifies a subject through a known relationship. The next section addresses indefiniteness of the subject (tankīr al-musnad ilayh): why might a speaker deliberately leave a subject grammatically indefinite?
The issue is not limited to “known versus unknown.” Indefiniteness may point to an unspecified individual, a particular kind, or may contribute to magnification, disparagement, multiplication, or diminution.
In «جَاءَ رَجُلٌ», the subject is one unspecified man. In «عَلَى أَبْصَارِهِمْ غِشَاوَةٌ», the expression can evoke a kind of covering with a particular quality. Calling both merely “unknown things” would miss important rhetorical distinctions.