no. they were just something everyone in the industry had on hand all the time when writing code back when I started. It was just understood you would have one..
When to use one: if you want to see what is in a 'binary' file.
How to use them: there is really only one rule: make a backup of the file before you change it. Because that byte you changed may not be what you thought it was. Change something, test it carefully, making small changes at a time.
they are mostly self explanatory. you have something like this
0A 12 FF 34 18 32 B4 ..... and on the side, the ascii for it: W H A T E V A (unprintables have a placeholder symbol)
and usually on mouse over or in another window/section, the integer values of your current mouse position as 8, 16,32,64 bit ints (signed and unsigned) and float, double, and (some of the) long double formats. Long double is getting weird .. there are 10, 12, and more byte versions now, I don't know which editors support which setups.
Typically you are looking for something: a text string, a specific value, etc. For example, a character in a game file might have 5 or 6 stats, 15 13 10 18 ... so you can look for a 15 13 and see if the next one is 10, and if you find that, set that 10 to 20 or whatever to cheat, save it, try it, see if it worked or not... etc.. editing executables to reroute program flow takes a great deal of understanding and patience. Changing a value or two is simple and the best place to start. I can't remember the last time I actually edited a file with one. I think it was to repair a file that was damaged for my wife. I LOOK once in a while, but rarely touch. A good use of LOOKING is to try to reverse engineer a file format, or at least partially, so you can open it in a program or something, or get past a documentation mistake/omission when dealing with a file format.
drag and drop a file into visual studio used to pop into a hex editor built into it. It wasn't a great one, but you can get a quick 'this is what they do' from it.