Project keys are used as identifiers to make it easy to know the project in which a work item is associated. Here's an example: Let's say you give your project a key of "ABC". Each work item created inside that project will inherit the project key as its work item key (yes, work items also have keys) followed by a number that increments automatically. So, the first work item added to that project will have its key set to "ABC-1", the second one will be "ABC-2", and so on. Keys become useful when you have similarly named work items. Sometimes the key is the only way to differentiate them.