The post about geo-tagging photos from a GPS track featured an app using .gpx files. .kmz files are useful for working with the Google Earth app.
One use of .kmz files is tor share information among people who have Google Earth installed on their computers and mobile devices. Open this small (124K) .kmz file in Google Earth to see the iPhone GPS track of a day of driving and hiking in the Ekalaka Hills and the Chalk Buttes of southeastern Montana. I kept my GPS app running for the entire drive so I could later geo-tag photos I stop to take along the way, either from the car or on foot, with the simultaneously-recorded .gpx file. I plug my iPhone into power while driving to keep it charged between hikes.
You can also create a .kmz file in Google Earth to share. This even smaller (2K) .kmz file of some 2012 wildland fires in the same area was created with Google Earth Placemarks, then saved as a Place.