We use it to ease the creation of our quarterly billings. In short, we do them this way:
Listing the cases we need to decide on whether or not to bill them by picking our FogBugz "quarterly billing - preparation" filter. This filter searches for all cases in customer projects that do not have a special quarterly billing tag.
Adding these special quarterly billing tags. We create tags like "to-bill" and "goodwill", drag the cases from the left to the tags at the right and save the tags on them.
Listing all cases to bill by picking our "quarterly billing - to bill" filter. It searches for all cases with a "to-bill" tag and without a bill identifier tag.
Putting these cases on the bills and marking them as billed. We create a new tag per customer with a bill identifier as it's name. When we drag the cases from the left to the right, a "copy summary" button shows up we use for copying the title and elapsed hours to our billing software. Then we save the bill identifier tag.