CLEO allows you to organize individual extensions/themes by combining them together into a single, installable file. This file can be used to install or restore a collection of extensions and/or themes.
You may have a list of your favorite extensions/themes that you would like to share with others. With CLEO, you can gather them all together into a single file (called, perhaps, "My favorite add-ons") and post it on a webpage or email it to family, friends and co-workers. They can install the whole batch with a just a couple of clicks! (e.g., File > Open File…)
Useful tool for developers: I, as an extension developer, have several different Firefox profiles. Whenever I start a new extension, I create a new profile for testing, debugging, etc. In this profile, I like to have a dozen or so developer related extensions installed. With CLEO, I can package all these extensions together and install them into a new profile with just a few clicks.
Once installed, CLEO is available from the Firefox Tools menu. An icon can be placed on the toolbar via (right-click on toolbar) > Customize...
*Available elsewhere on this Mozilla site