You come across an undescript binary. It might be a jpeg with an altered extension, malware or basically anything. You open it in a hex viewer and try to gather some info.

Your lab manager goes "hey, can you generate these proprietary binary files (currently made with a horrible gui) to control this robot programmatically".

This isn't very common, but I was trying to build a common CLI tool in the browser to use in an internal application at work. I compared output from my implementation with the CLI tool until I got to feature parity (and to clear up confusing things in the spec/source code).

For other things, I've used it for examining network packets (binary protocols), mystery files, and proprietary data formats. I've also used a hex editor to hack executables (most notably hacking in a dark theme into Unity3D).

One (of many) uses I used xxd for: viewing a byte stream that is supposed to be utf-8 text, but the non-ascii characters in the file are not displaying properly. Hex viewers will let you see exactly what bytes are present, without interpretation. As for the utf-8 stream, it turned out to have been double encoded into utf-8.

So i have been working on an embedded project lately where i had to export the framebuffer to an sd Card. I used a raw Format first (netppm) and had to use vim in hex Mode to validate the header. So this is really neat. I was looking for an padding option in the readme but i guess i ll have to install it later