An ini file is a structured format of plain text file. If you open an ini file in a text editor it will be human readable, however it won't just be laid out as written text, it will have topics and item names that allow programs to easily find the peice of information they are looking for.
/writeini could probably write to a text file, but it would write to it in an ini file structure, eg.
[topic]
item=value
Obviously you wouldn't want that in a regular text file since it would have no meaning.
