They are a storage bin for everything that can happen for a particular event. It could be something simple and reasonably short-term like a customer enquiry, or it could be long and involved like preparing for all the requirements for having a stall at a trade show. It holds all the relevant information and history in a common place so everyone can see it, and where necessary contribute.
There are two places to activate a case:
Tracks behave the same as they do in other places. When you select and start a track to run in the case all tasks will appear on your to-do list in sequential order. If at any time the case is closed, the tasks will be removed from your to-do list and the track will display in the case.
It cancels all tasks, including those in a track. The history is retained and you can continue to review this information against the party record and in lists created in the Cases window.