Voiced aspirated stops are one of the features that made Sanskrit central to the study of language in nineteenth-century Europe. Most of the older Indo-European languages had lost the series, merging it into plain voiced stops or turning it into fricatives, and Sanskrit had kept all four. When scholars began comparing Sanskrit with Greek, Latin and the Germanic languages, the regular correspondences between these letters and quite different sounds in each of the others were among the clearest evidence that the resemblance among the languages was systematic.
Middle Indo-Aryan kept the voiced aspirates too, where it lost a good deal else, so the letter is common in the Prakrit of the edicts rather than being a refinement of the later Sanskrit-writing script. That is fortunate for the palaeography, since a letter attested in quantity in the earliest material gives a firm anchor for the sequence of its shapes.
The phonetic tradition is emphatic that the sound is a single articulation and not a stop followed by a separate breath, which is why Brahmi writes it with one letter. Speakers of languages that lack the sound reliably hear it as two, and the two-letter transliteration used in European scholarship encourages the mistake. The Indian analysis rejected that reading two and a half thousand years ago, and modern phonetic description agrees with it.