Archiving moves posts out of the main posts table into an archived posts table. This can make the forums_posts table smaller, which will improve performance for any sort of action that hits the posts table.
i.e. instead of 10,000,000 posts in the forums_posts table, if you end up with 4,000,000 and 6,000,000 in a backup table, then any action against the main posts table will undoubtedly perform faster.
The forums_archive_posts table can even be stored in a completely separate database if appropriate for your environment.