An indefinite predicate may avoid tying an assertion to an already identified individual description. In other contexts, indefiniteness can suggest magnitude or minimization. There is no single automatic rhetorical meaning for every indefinite predicate.
«زيد كاتب وعمرو شاعر»
Zayd is a writer, and ʿAmr is a poet.
«هُدًى لِلْمُتَّقِينَ»
A guidance for the God-conscious. (Qurʾān 2:2)
The first example simply attributes qualities. In the Qurʾānic context the indefinite «هدى» can underscore the greatness and significance of guidance.
The intended force of indefiniteness is selected by contextual evidence: an unqualified quality, magnificence, or another identifiable shade of meaning.