The ed editor is a very ancient and primitive editor. This was all there was back in neolithic times, but now its simplicity is sometimes useful when performing operations on files from scripts when the load times of Vim and other modern editors are too long.

Deleting A Line In Place

For me this is the most common modern simple task for the ed editor:

printf "%s\n" "10d" "w" "q" | ed -s thefile

The printf sends a sequence of commands followed by newlines to the ed program which acts on thefile. In this case line 10 gets deleted, the new file (missing its former line 10) gets written (w) and then the program is quit (q). The -s, for silent, makes ed suppress its cryptic and terse output which is definitely not needed for this script.

Note
 Modern sed implementations can often do in-place editing with -i flag.