> I knew about 'ed' but never really understood what made it special or where/how you could use it.

ed script is one of the formats supported by diff and patch (with their -e or --ed command line switch). (diff generates the script by itself, but patch just pipes it to ed.)

ed scripts are used at Apple to maintain their patches for Python: see e.g. https://opensource.apple.com/source/python/python-97.50.7/2.... and others in https://opensource.apple.com/source/python/python-97.50.7/2....