So, the answer lies in how Evolution stores contacts. If you’re running Evolution, you’re actually running two pieces of software – the Evolution client, and the Evolution Data Server (EDS). The client is what most users think of as Evolution – it’s the program with the GUI that allows users to access and manipulate their mail, contacts, calendar, etc.
But the heavy lifting is really being done by the Evolution Data Server. The EDS is the program communicating with the e-mail servers to get and store your mail. It’s the program communicating with your various address books (local or remote, like Google Contacts). It’s the program that’s communicating with all of your calendars and task lists.
Essentially, if your Evolution experience was a restaurant, then the Evolution Data Server is the kitchen. The Evolution client is just your waiter. The waiter takes your orders and passes them off to the kitchen, the kitchen does all of the cooking, and then the waiter brings you the tasty results.
The fact that GNOME has split Evolution like this is really handy. It means that alternative clients can access Evolution’s mail, contacts and calendars. Essentially, the user could uninstall the Evolution client, but keep EDS installed, and still have access to all of their mail, contacts and calendars.
To stretch the restaurant analogy a little bit, what I’m saying is that the GNOME developers made it possible for you to order take out from the Evolution restaurant via a third-party.
So what’s the advantage of that?
Well, for one thing, Evolution Data Server talks to some services that Thunderbird doesn’t – for example, Google Contacts (although, this extension suggests that Google Contacts integration for Thunderbird is possible without EDS).
The other big one is Ubuntu One contacts sync. Ubuntu One contacts are stored in CouchDB address books that are accessed through EDS. Now that Thunderbird can read your EDS address books, it means that you get your Ubuntu One contacts as well (at least, it will be able to, once Ubuntu One contacts sync starts working in Oneiric).
Not bad, eh?