VI is one of the most used text editors in the UNIX world. It is found on most (if not all) UNIX workstations, and not only on Linux ones , contrary to most other text editors.
It is very powerful, which means it can be hard to do simple things, BUT it also means that complicated thing are equally hard to do. So if you manage to understand how VI works for simple things, you can quickly move on to very advanced text editing.
[Besides, it looks awfully cool to edit text in vi, rather than in emacs or other text editors that actually display command keys, or even have menus :-)]