Even in binary you can see patterns. Not saying it's perfect to show binary diffs (but it is better than showing nothing) but I know even my slow mammalian brain can spot obvious human readable characters in various binary encoding formats. If I see a few in a row which doesn't make sense, why wouldn't I poke it?
What should I look for? The evil bit set?
Sure, the same person who's gonna be looking is the same person who'd click "show diff"
This particular file was described as an archive file with corrupted data somewhere in the middle. Assuming you wanted to scroll that far through a hexdump of it, there could be pretty much any data in there without being suspicious.