I believe xxd is actually part of vim and not a Unix command itself. It's used to support hex-editing binary files, but you can see why it's useful in other roles as well.
$ man xxd | grep vi
Use xxd as a filter within an editor such as vim(1) to hexdump a region
Use xxd as a filter within an editor such as vim(1) to recover a binary
Use xxd as a filter within an editor such as vim(1) to recover one line
From what I've been able to find, vim was released in 1991, and xxd has been around since at least 1990, whereas vi was written by Bill Joy in 1976. (according to Wikipedia)
Interesting bit of trivia, perhaps someone with a good memory can clear it up for us?
I'm a bit more tolerant of vi now, but just because I "never" see it courtesy of suitable EDITOR entries.