Recommended Threads was created to be used instead of the similar threads function of vBulletin. Rather then matching on the title, Recommended Threads matches the body of the first post and gives you threads relating to the real post data.
Why use Recommended Threads?
The out of box similar threads function of vBulletin only matches on the title of the thread. This can lead to having similar threads that have nothing to do with the body of the post. Using Recommended Threads will show threads that are more likely to be related to the post they user is reading.
Recommended Threads has low overhead since the data is stored with the thread you are reading. Every hour new threads will have their recommended threads created and every night any thread that was read that day will also have their recommended threads updated.