The Dravidian languages have a retroflex nasal independently, as a sound in ordinary words with no conditioning rule behind it, so the letter is common in the southern use of the script from early on. That is the general situation in the far south: the retroflex series that Indo-Aryan acquired partly from its neighbours is native equipment in Dravidian, and a script carrying the full class arrived in Tamil country already supplied with most of what the language needed.
What it did not supply were the three additional consonants that Tamil-Brahmi added at the end of the order, one of which is an alveolar nasal held apart from both the dental and the retroflex ones. Three nasals at three points of articulation is a distinction Indo-Aryan does not make anywhere, and the extra letter is one of the clearest signs that the southern use of the script is a fitting of a writing system to a language of a different family rather than a regional variant of the northern one. The retroflex nasal itself was received rather than added, and it keeps its inherited place as the fifth member of the third class.
For a reader of southern inscriptions the practical effect is that the letter appears where a Sanskrit reader would not expect it, in ordinary words with no retroflex or r earlier in them to have conditioned it. That is a useful signal about the language of a text: a retroflex nasal without a conditioning environment points away from Indo-Aryan. The standard handbooks list it among the features by which the language of a southern inscription is identified before its content is interpreted.
The same holds for the retroflex lateral, which the Dravidian languages use freely and which Indo-Aryan needs only in Vedic recitation and in a handful of dialects.