I definitely need to edit files that are hundreds of thousands of lines once in a while. Nothing wrong with it, I think you're just assuming it's source code or such when it isn't. It's often data manipulation (CSV, JSON, etc.) and it feels like torture when the editor can't handle it. I've even opened multi-gigabyte text files in my editor and needed to edit them sometimes.
That's cool, understood. Whilst I don't think you would be using my app to edit those files, it could do it. Though I haven't tested the multi gigabyte size on iOS.