Probably something completely unnecessary. I asked in the technical forum issues forum and got a vague response. The very fact that vBulletin is built on PHP+MySQL is a sign that it's probably a piece of garbage. Then again, I'm not sure there are any forum packages that are very well written. It's a sad state of affairs.
My first guess would be backups. vBulletin seems to rely on MyISAM for full text searching, so this means that you can't have transaction support. Without transactions, you can't reliably back up a table without locking the table, preventing any writes to it. It's stone-age technology. I have to suffer with MyISAM in my day job and I'm pretty bitter about it shortcomings.
The forum could possibly still function in read-only mode, but I don't know if vBulletin supports such a thing. Even viewing a thread causes writes (incrementing the view count).