That table is for the fulltext index. XenForo maintains a separate table with copies of the posts for the fulltext index rather than having the index as part of the original post table. This is probably because the post table is InnoDB. If XenForo had MySQL 5.6 as a requirement then they might change that. In the meantime space is cheap and large forums will be using the Enhanced Search addon anyways.
It would be nice if we could have the option of adding FT indexes to the main InnoDB thread/post tables at some point (for those of us on 5.6). I understand it may not be soon, but I'd very much like to save some of that storage space on a few of my forums (SSD RAID setup, so not as cheap storage as I'd like).
I've got FT InnoDB search working on another forum software (take a guess which ) and it works well. For medium sized forums that aren't search intensive, it's more convenient than the ES addon (especially on my Windows server).