In some situations explicit mention is needed for clarity; in others the preceding discourse makes omitted material recoverable without repetition.
مَنْ حَضَرَ؟ — Who came?
زَيْدٌ — Zayd. The remainder of the answer is recoverable from the question.
حَضَرَ زَيْدٌ — Zayd came. The fuller sentence is also valid.
The abbreviated answer illustrates economical ellipsis. But outside the question-and-answer setting, a bare “Zayd” may not express the intended proposition. Mention and omission depend on context and communicative need.