Rated 4 out of 5 stars

What's the advantage of using this plug-in versus natively using Google's CalDAV prototcol to access my Google calendar?

I don't get yet why one would need this plug-in ???

This review is for a previous version of the add-on (0.5.1). 

Google only recently added CalDAV access to Google Calendar. This plugin was created back in 2006/2007, where there was no other possibility than to use the API. I am also working on improving the CalDAV provider to work with Google, eventually this addon will provide any extra configuration options usually not available with CalDAV calendars (i.e adding guests and settings ACLs)

Also, Google's CalDAV support is not considered production ready, last I looked.