At the start, the two subwindows are pointing to identical spots in the file. But I can move throughout the file within the two subwindows, indpendently of each other. A good editor also allows re-do, which means that if you undo something but then decide you want it after all, you can retrieve it. Instead, use the auto-indent mode. In this mode, whenever you hit Return to start typing on the next line, joe automatically will indent to the same amount as the previous line.
Most good editors offer such a feature.
Say you are debugging a program and have just fixed a bug in a file, say g.c. You now want to compile it. First you must save the file, but you don't want to leave the editor. Most good editors include commands to do this. Many editors also allow you to coordinate your compiling with your editing.