Whether a descendant of Brahmi carries this letter is a straightforward statement about the language it serves. Marathi, Gujarati and Oriya keep it because they use the sound. Standard Hindi does not, and its alphabet chart accordingly omits it, though the same Devanagari script writes it when used for Marathi. Kannada, Telugu, Tamil, Malayalam and Sinhala all have it, since the Dravidian family uses the sound throughout and Sinhala shares the areal feature.
That variation makes the letter useful as a marker. An alphabet chart is a compact statement of what a language needs, and the presence or absence of one letter at the end of the consonants tells a reader something about the sound system without any further inquiry. The same test applies to the three Tamil additions, which are found only in the alphabets of the far south, and to the dotted letters of the north, which are found only where Persian and Arabic vocabulary entered in quantity.
Brahmi had the letter from early enough for it to be inherited widely rather than reinvented, which is why the scripts that have it agree on where it sits. Where a feature is inherited the descendants agree; where it was added independently they do not, and telling the two cases apart is one of the ordinary problems of comparing these alphabets.