From looking at the way some forum softwares are storing data in a database (eg. phpBB uses MySQL databases for storing just about everything) I started to wonder why they do it that way? Couldn't it be just as fast and efficient to use.. maybe xsl with xslt to store forum topics and posts? Or to at least store the posts in a topic?
There are loads of reasons why they use databases and not flat files. Here are a few off the top of my head.
Indexes and efficient searching
Here are a couple more posts you can look at for more information :
If i can store my data in text files and easily can deal with these files, why should i use database like Mysql, oracle etc
Why use MySQL over flatfiles?
Why use SQL database?