There are also "similar threads" suggestions when you start a new thread. As you type the title of your OP, you get a list of similar threads pop up below your title based on the keywords you're typing. They can be ignored. Just keep typing and go about your business as usual.
The reason for this feature is to show a person similar topics that might answer their question or even reveal if a discussion thread already exists for that topic. Some forums are really bi_chy about people asking questions that have been asked already (reddit's DIY_Ejuice comes to mind

) so this feature is a welcome feature for some forum owners.