Rated 5 out of 5 stars

does what it says but unfortunately I am finding it hard to actually grasp what the real purpose of this extension is?

This review is for a previous version of the add-on (3.0). 

The general purpose is to give users more control over the way in which web content is presented (see also Column Reader, one of my other extensions). Colorific may be used by web designers to easily test alternate designs, but average users should also find many applications, such as preparing web content for use in presentations or simply for reading in the dark using a light-on-dark color scheme.