The prupose of this module is to pre-generate styles via rules.
DO NOT USE THIS IN PRODUCTION. THIS IS AN EXPERIMENTAL MODULE.
Normally the image derivates get created on request. But what if you want to display informations on presets, without displaying them. Like a download link with an info on size, resolution...then the image must already exist.
This module is a plugin for the great Rules module
So you can use rules to only run it on special node types and actions. I personally use it to generate the styles on "node save and node update".