أما حذفه فللاحتراز عن العبث بناء على الظاهر
The subject may be omitted to avoid what would appear to be needless repetition.
If context already identifies the person under discussion, repeating the subject can add little. In reply to “كَيْفَ أَنْتَ؟” (How are you?), “عَلِيلٌ” (Unwell) can convey “أَنَا عَلِيلٌ” (I am unwell), because the question supplies the missing subject.
Omission depends on an intelligible contextual clue (qarīnah). Without that clue, brevity can become ambiguity. The rhetorical advantage arises when the hearer can recover what is left unspoken.
An additional purpose is to engage the hearer’s attentiveness and inference when the contextual clue is sufficient.