They don’t serve one. Clojure uses only white space for distinguishing items in lists and hashmaps and after a short period of “this is weird” you embrace it and wonder why you ever needed commas at all
Checksums, basically, drawing a line between random bytes and structured JSON in a way that comes at zero cost for a human mind (arguably less than zero). We might as well leave out all closing tokens at the end of the stream, in a way those are completely redundant but still worth it for the warm, fuzzy feeling of knowing that you haven't just read a truncated stream.