I work in videogames and we often use custom binary file formats, a HEX editor is very useful because, as an industry, we don't document anything and the code is often a car crash :)

What title are you referring to? I am guessing that title got edited and initially used partial header from github page "A Hex Editor for Reverse Engineers" cutting of second part "Programmers and people who value their retinas when working at 3 AM".

Hex editor can also be very useful for programmers when working with binary file formats. Implementing support for image and archive formats. Testing that your code outputs the thing you expected it to output or inspecting a file which your program fails to process because some other program included some unexpected option fields or something like that. It's sometimes even useful even when working with text fails, typically because they included some invisible symbols which you didn't expect or differentiating between similar looking unicode symbols.

How else are you supposed to debug misbehaving byte order markers and emojis and other U̷̼͌T̷͔̑F̷̤̂ ̴̯͗b̶̤̋l̵̜̈́a̸̭̚c̵͕̊k̶̦̅ ̶͓͆m̸͙̎a̴͖̿g̴̠͝i̸͛ͅc̸̡͌ in your "plain text"?

At work, I sometimes have to encode or decode binary data, to which I have full specs, and a hex editor is an extremely valuable tool.

A hex editor is to binary data what a debugger is to code, a packet analyzer is to networking, and an oscilloscope is to electronics. These tools can be used for reverse engineering, but it is certainly not their only purpose.

I don’t work with binary files much these days but I still use hex editors a lot just to debug any weird rendering glitches in terminal emulators. The ability to see the exact character codes being generated rather than the terminal emulators interpretation of them is invaluable. Though for that purpose hexdump is usually sufficient.

Back in the 90s, when I was working with binary data regularly, I’d have killed for something like ImHex.

If you're writing tools that generate spec-based binary files, you need a debugger that lets you examine generated files at the binary level. For instance, I don't think you can do any OpenType font tool engineering (as in, working on applications that generate fonts) without having a good hex editor to check whether the binary internals of your files are structured correctly.