This plugin basically just saves all of the various tiny queries to build a page as 1 large query. So instead of running 25 small queries per page, it runs 2 or 3 bigger queries. For certain types of sites (CMS sites with small number of pages) this can greatly improve performance.
The memcache plugin for percona is actually the reverse of what most people probably think it is (at least what I thought it was). It doesn't add caching functionality to MySQL - rather it adds MySQL persistance to memcache.