I was under the impression BARD only had the core books in audio format.
Anyways, I don't use braille, so have no clue what, if any, advantages BRF has over other formats, but as far as I'm concerned, having more formats available is generally a good thing... I for one, would love if more eBooks were available as ascii plain text with separate files per chapter/other divisions and don't particularly care for any of the widely used eBook formats.
That said, my understanding is Wyrmworks wants to improve accessibility in the TTRPG space across the board and I suspect the SRD was picked as a first target for conversion for Braille because its something both highly relevant to the hobby as a whole and which wouldn't require a licensing agreement to put out there for feedback from those who would find such conversions useful.
As for it being listed under the PDF category, I bet gold to silver the Drivethrough folks just assume most eBooks offered on their service are going to be PDFs and they just don't have a generic eBook category... that, or they have categories for other common formats, but no "other formats" category.