Firs of all, changes must be in all, otherwise, if I modified one part in the 1sh window, and another part in the 2nd window, then when I save each of them the 2nd save overwrites changes from the 1st.
Why would I open several windows?
So I can scroll them to different locations. If I work on big files, then I often need to see several locations in the same file.
This also applies to seeing 2 different files side by side.