Text editors are provided with operating systems and software development packages, and can be used to change configuration files, documentation files and programming language source code.
From Wikipedia again:
Richard Stallman founded the GNU project in September 1983 with an aim to create a free GNU operating system. Initially the components required for kernel and development were written: editors, shell, compiler and all the others
From these quotes, it seems that a text editor is required in an OS.
When I used Windows, there were only a few occasions that I had to actual edit any system files myself, and I would have to believe that the average user never ends up opening a text editor to edit system files.
In Unix/Linux, and maybe Windows, you can edit a text, such as appending, file using commands without even opening a text editor, so a editor would not even be necessary. I'm not sure if you can put text in the middle of a file, so perhaps that's where a editor would come in place.
Is it the operating system that uses the text editor in the background for system uses? So, whenever a file needs to be changed, the OS will in the background invoke something such as
$emacs someFile and then edit it, without us seeing, if there is some way to hide it.
Update: Are the downvotes because it's a very obvious answer, or because the question is inappropriate?