In the beginning, XenForo Enhanced Search (XFES) was intended to be a simple drop-in replacement for MySQL full text search, primarily aimed at large forums whose operation would potentially suffer when attempting to search very large database tables.
As a drop-in, XFES only made use of the enhanced search index and speed for the formal search system, but with version 2.2 we are starting to leverage more of the power afforded by the Elasticsearch back-end.
Similar threads
A useful tool for surfacing content on your forum is a list of Similar or related threads, which can be displayed along with a primary thread. Visitors who have landed on that page from a search engine will often look for related material to continue their reading...