The first purpose is ifrād: referring to some individual to whom the common noun applies without making the individual's prior identity part of the utterance.
وَجَاءَ رَجُلٌ مِنْ أَقْصَى الْمَدِينَةِ يَسْعَى
A man came running from the farthest part of the city. (Al-Qaṣaṣ 28:20)
«رَجُلٌ» is indefinite. Whether the man had a real personal identity is a different matter: the sentence first draws attention to his arrival and action, rather than recalling him by a known designation. Ifrād means one relevant individual is in view, not that the individual must lack a name.