وَفَائِدَتُهَا: تَخْفِيفٌ فِي اللَّفْظِ فَقَطْ.
Translation: Its benefit is solely ease or lightness in expression.
Compared with ضَارِبٌ زَيْدًا, the construct ضَارِبُ زَيْدٍ drops tanwīn and is structurally more compact.
In the book’s analysis, this formal construction does not automatically confer definiteness or specification in the same way as semantic iḍāfah.