After omission, the discussion turns to the purposes of mentioning the subject (ذكر المسند إليه). Expressing it is the basic form of explicit predication, but in a particular context it may carry further rhetorical benefits that would disappear with omission.
وأما ذكره فلأمور
There are also several reasons for expressing it.
فلكونه هو الأصل، ولا مقتضى للعدول عنه
Because it is the basic form and there is no reason to depart from it.
At the start of an exchange, «الْعِلْمُ نَافِعٌ»—“Knowledge is beneficial”—is more informative than a bare «نَافِعٌ», since the listener has yet to be told what is beneficial. Later, with the subject established, the choice may differ. The default does not mean that expression is invariably superior to ellipsis.