Vim is free and open source software and is released under a license that includes some charityware clauses. It was written by Bram Moolenaar based on source for a port of the Stevie editor to the Amiga and first released publicly in 1991. Thanks Wikipedia ! However, we still don't know why you should use Vim (or just Vi).
"Why should I use Vim ? Now we have so many powerful and easy tool ! "
It is free !
Yeap ! It is free, no need to pay if you prefer ! And this is big point.
It has a huge active community
Just tap "Vim" in Google and see all the websites who talk about Vim, Vi or VimScript.
You can use it on all UNIX platform (MacOS included)
Yeap, everywhere ! Vi est everywhere Vim is optional, it is more or less the same command, you won't be lost anyway !
Very simple to use
No joke, here you can find my tuto about Vim/Vi where I introduce only several commands who alow you to use this editor correctly and do, more or less, everything on every UNIX system and no needs to have a Master degrees to understand it !
I think I can say without any doupt then Vim is the most customizable editor, from my point of view.
You can install hundred of addons
No needs to use mouse, keyboard only
I said, no need but you can actually, as an example, I am using mouse to copy/paste, it is not real-vim-user compliante ...
Can be deployed everywhere
Light version for an embedded system, heavy version for a web server or your desktop, you can manage your Vim package for each specifique area with Vim you are able to modify any kind of files on any platform everywhere.
Who it works
Vim are using modes, you can't write text if you aren't in insertion mode or stay in default mode in order to copy/paste, move the cursor, ... and others commands like find, go to, next iteration, plugins, ...
Works in a simple Terminal
No needs to have a huge CPU to be able to have Vim working find, no useless effect or interface issue.
Bonus: Using Vim mean "I control my project !"
It is kind of philosophical but it is true ! When you are using Vim you are alone face to your code/project. You have to undestand each line and know precisely what you want to do and why, not try random stuff until it works (well ... I have to admit I already did it ... like everyone isn't it ?).
Vim can be very useful and something inevitable, so make your choice, become a Vim-user or a least learn how to use Vim for specifique and precise operations.