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?

Sure, the same person who's gonna be looking is the same person who'd click "show diff"

What should I look for? The evil bit set?

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.