Why would Vim add a new line at the end of a file?

I work with Wordpress a lot, and sometimes I changed Wordpress core files temporarily in order to understand what is going on, especially when debugging. Today I got a little surprise. When I was ready to commit my changes to my git repository, I noticed that git status was marking one of Wordpress files as not staged for commit. I remember I had reverted all the changes I did to that file before closing it, so I decided to use diff to see what had changed. I compared the file on my project with the file on the Wordpress copy that I keep in my downloads directory. It turns out the files differ at the very end. diff indicates that the there is a newline missing at the end of the original file:

1724c1724
< }
\ No newline at end of file
---
> }

I never even touched that line. The changes I made where somewhere in the middle of a large file. This leads me to think that vim added a newline character at the end of the file. Why would that happen?

All the answers I've seen here address the question "how could I prevent Vim from adding a newline character at the end of the file?", while the question was "Why would Vim add a new line at the end of a file?". My browser's search engine brought me here, and I didn't find the answer to that question.

It is related with how the POSIX standard defines a line (see Why should files end with a newline?). So, basically, a line is:

3.206 Line

A sequence of zero or more non- <newline> characters plus a terminating <newline> character.

And, therefore, they all need to end with a newline character. That's why Vim always adds a newline by default (because, according to POSIX, it should always be there).

It is not the only editor doing that. Gedit, the default text editor in GNOME, does the same exact thing.

Edit

Many other tools also expect that newline character. See for example:

• How wc expects it.
• GCC warns about it.

Also, you may be interested in: Vim show newline at the end of file.

Because vim is a text editor, it can sometimes "clean up" files for you. See http://vimhelp.appspot.com/vim_faq.txt.html#faq-5.4 for details on how to write without the ending newline, paraphrased below:

How do I write a file without the line feed (EOL) at the end of the file?

You can turn off the eol option and turn on the binary option to write a file without the EOL at the end of the file:
   :set binary
   :set noeol
   :w

Alternatively, you can use:
   :set noeol
   :w ++bin

Adding a newline is the default behavior for Vim. If you don't need it, then use this solution: VIM Disable Automatic Newline At End Of File

To disable, add this to your .vimrc

set fileformats+=dos