Ahh Edlin. Used it a fair bit in the 80s, until I discovered Sidekick, and got a pirate copy.
I used to program a lot and needed a text editor, sometimes for notes, and sometimes to update actual code. I didn't really use Edlin for notes, but I used it to edit source code.
Sidekick was awesome though. In these days of computers being able to run hundreds of tasks simultaneously, it's easy to test something and switch back to your programming IDE to make updates.
That wasn't an option in DOS. If you needed to update source code, you would have to quit your application, make changes,possibly re-compile then start the test again,
Sidekick doesn't entirely remove that problem, but it makes it easy to switch to your source code, edit it, then when you are ready, you can quit the application, recompile and test again.
But Edlin was a good emergency backup.. If you want to see a rough guide to Edlin, this video shows Edlin for Freedos, but it's near enough. https://www.youtube.com/watch?v=CIlJeKuSl9w