The Sanskrit root meaning to give begins with this letter, and its derived noun, the word for a gift or a donation, is the single most important word in the history of reading Brahmi. When James Prinsep worked out the values of the letters in 1837 he did it not from the great Ashokan edicts but from the short records cut into the railings at Sanchi, dozens of them, each naming a donor and closing with that word. A formula repeating with only the names changing is exactly what a decipherer needs, because the constant part can be guessed and the variable part then tested against it.
The same root supplies a great deal of ordinary vocabulary, and the giving of gifts to religious communities is the subject of most of the earliest Brahmi inscriptions there are. The donative record is the characteristic early Indian inscription: a monk, a merchant, a guild or a queen gives a pillar, a railing, a cave or a cistern, and the fact is cut where the gift stands. Thousands survive.
That means the corpus a palaeographer works from is heavily shaped by a single social practice. What early Brahmi records in bulk is not literature, law or narrative but the naming of donors, and the vocabulary of the surviving material is correspondingly narrow. Letters that occur in the donative formula are attested in thousands of examples; letters that do not are far harder to date.