I’ve known about and used ed(1) on many occasions. Can save you in a pinch when all you have access to is /bin in single user boot mode. Even learned a couple of new things I didn’t know like chaining g and s commands. Once found myself on one of those other *nix that had no ed(1) nor sed(1), but had an ex(1) that I could use in a script to automate some configuration file edits.
Certainly worth knowing the old tongue of editing along with the Charm of Making, “Anál nathrach, orth’ bháis’s bethad, do chél dénmha”.