The ordinary Sanskrit negative is this letter with its inherent vowel and nothing else, a single sign standing as a word. It negates a verb, a sentence or a clause, and it is among the most frequent words in the language. A reader of inscriptions learns to spot it early for the same reason they learn the word for and: a lone sign in a stream of longer groups is easy to see, and the meaning of a sentence turns on it.
The letter is also the commonest nasal in these languages and, unlike the velar and palatal nasals, it begins words freely. It forms the past participle of a class of verbs, appears in a wide range of nominal suffixes, and stands in the endings of several declensions, so it is met constantly at both ends of words. In Sanskrit it is the source of most retroflex nasals, through the rule that converts it when certain retroflex or r-like sounds occur earlier in the same word.
That rule is one of the more remarkable pieces of the classical grammatical analysis, because it describes an effect operating across intervening sounds rather than between neighbours, and the tradition states exactly which consonants block it and which let it pass. A reader who knows the rule can often reconstruct a damaged syllable from the nasal that follows it, which is a practical benefit to an epigraphist and not merely a point of theory.