Do you have a big MODX website? Like, thousands of resources and lots of stuff in the templates? Are your pages taking several seconds to render when they aren't cached? When search engines start crawling a site like that when it's not fully cached, it may become extremely slow. So you can't afford cleaning the whole cache every time you save a resource.
This module provides a different strategy: when you save a resource (or its pub_date time comes), it clears and regenerates the cache of this resource, its parents and the index page (site_start system setting).