The aspirated labial is not a common sound, but one of the words it begins is among the most consequential in Indian religious vocabulary: the word for fruit, used from early on for the result or consequence of an action. The doctrine of action and its consequence, which runs through Hindu, Buddhist and Jain thought alike, is stated in terms of that word, and the technical literature of every one of those traditions uses it constantly. A term for the ripening of a result is built on the same root.
Beyond that family the letter is relatively rare. Aspirated labials are less frequent than their unaspirated partners at every stage of the language, and a text of moderate length may not need the sign at all, which puts it among the less well attested letters in the early inscriptions. It keeps its place regardless, second in the labial class, because the position follows from the analysis rather than from the count.
The sound is not the f of English, which classical Sanskrit does not have. Several of the northern descendant scripts later took this letter and added a subscript dot to write the f that arrived with Persian and Arabic vocabulary, filing a new sound under an existing letter rather than extending the order. Brahmi itself has no such marks and no such sound.