When creating KML files when is it correct to use the compressed format of KMZ and when should you just leave them umcompressed?
There are advantages to both formats and the line between them can be gray in places but there are some instances in which you have no choice but to use a KMZ.
Times when you have to use KMZ
If you want to include images, models or custom style icons but can not/do not wish to host them on web you have to add them into a KMZ file.
Times when you should use KMZ
If your file contains a large number of features it is best to compress it into a KMZ as this will significantly reduce the size and therefore the download and load time for your users.
Advantages of KML
The major advantage of plain KML is that it’s easier to use when it comes to manual editing. A KML file can easily be opened and edited with a text editor, such as Notepad - however with a KMZ, while you can still edit the doc.kml it contains, you have the extra step or zipping/unzipping and renaming. That said most zip programs allow you to edit files within the package without having to unzip it first - but it’s still simpler to use KML if your file is small.