When you connect to a server which is hosting a custom map, it is downloaded to your cache folder. The maps downloaded to your cache cannot be hosted, or played offline until they are converted in to playable maps and placed in to your published/custom maps folder. This is what this utility does. By launching the UT3CC you will be greeted with a list of maps currently contained in your cache directory, and by ticking the ones you want, you can then proceed by clicking the convert button. There is also a tick box underneath the list, which when ticked will empty the cache directory after it has converted and installed the maps for you.