I'm certainly not THE Garmin expert. And this is just some quick typing. Do not consider it to be anything definitive.
I've had a bunch of their units (among others) and I'm the type of person that likes to poke around things to see what will blow up - hopefully I learn from my experiences.
First of all let's assume that the DVD that came with your navigator contains the same version map that the Navigator has - this is important if you plan to chart routes and download them into the GPS. If the version is different the navigator will recalculate the route when it is invoked, based on the specified via ponts. If you have the unlock code and if the DVD map is newer than the one loaded in the avigator you can update the navigator from the computer.
The CNNT DVD contains several items. The keys are that it has a full map (assuming you have the unlock code). It also has a copy of Mapsource, Garmin's PC-based mapping program that allows you to interact with the navigator and the mapping.
Did you know that Garmin has additional software available for download on their web site? You can do screen grabs, design custom splash screens, define custom waypoint symbols and more with what they offer. Units that support Proximity POIs need a software package from the Garmin site to implement the feature.
CNNT installs on your hard drive - you'll use a couple of Gigabytes to host the map. Once that's done you can file the DVD away - it isn't needed for operation. I believe that the only maps that require the media are the US topographical ones - and even that can be faked out with a registry hack.
Once that's done, simply fire up Mapsource, figure out it's somewhat non-intuitive interface and away you go.
You can download the tracklog from your navigator; You can plan routes; You can upload supplemental maps that you can buy into your navigator and more.
However, most people route right on the navigator and if you don't have a computer or are intimidated by the prospect of some of these options you'll survive without.