There is no real reason to use edlin today (say who?), but it's still included with Windows XP. Enter edlin filename in the run dialog to experience its majesty for yourself.
Official documentation here:
Other documentation:
Despised though it was, EDLIN had one feature widely overlooked. It could be run in a batch file and do scripted edits. DOS batch file hackers made use of this facility in ways reminiscent of sed on Unix. --DMcCunney