What makes DokuWiki so great for Documentation?

Wikis are good for documentation

Easy to start: you can share unfinished content early, exposed to many eyes
Have a simple, consistent page layout
Easy to create and maintain structured, up-to-date documents:
- collaborative
- easy to access through the web, with simple login or without any login
- easy to modify, no need to know HTML
- easy to link pages
- easy to create simple and consistent pages
- easy to search
- easy to extend with external pages
Don't need to go to another colleague or IT dept to update the documentation = low resistance to keeping documentation up to date.

Easy to modify:
- section editing
Automatic services:
- future links: links to non-existing pages are red
- backlinks
- inner indexing
- revision handling
- table of contents for each page
Supports multiple ways of including code examples
Sophisticated but easily administrated access control through user-friendly ACL and User Management GUI
Simple but powerful Configuration Manager interface for control over features of not only DokuWiki core but also most plugins and themes. If you are not a PHP expert or don't have time to waste fiddling with conf – no problem!
Datafiles are stored in plain text, so:
- are readable even if your DokuWiki installation doesn't work anymore
- easy to back up, through server scripts or FTP/SFTP
- can easily be read, modified, compared, generated with external (non-wiki) programs
- and easily moved from one DokuWiki-installation to another
All those little gimmicks that make documentation fun: footnotes, abbreviations, syntax highlighting
Huge amount of available plugins for your specific needs
Maybe more I can't remember