When you need to specify what changes a service needs and don't know how to sort them out.
Identify changes to distinct features:
A feature is a user facing capability of a service that is:
- Triggered by the user.
- Returns a valuable result to the user.
Identify what changes should go together in meaninful packages.
A feature could be a baggage check-in at the airport, or a search in a web service. You approach the baggage check-in counter, leave your baggage, and receives a ticket. You surf to the search page, enter some terms, and receives a result list.
Note that the valuable result doesn't have to meet the user's need in full. A baggage check-in is part of a trip. A search result is part of whatever you want to do with the resulting items.
That is the idea behind packages. A package is a set of changes to a number of features. The changes won't meet a person's needs if they aren't in total deployed to the service.
There might be changes in the baggage check-in feature that needs to be in sync with other changes in the baggage flow. And maybe can you say the same about the search result. These changes must be identified and grouped in packages.
Constructs like these are common in service design, and have different names: user stories within epics, capabilities within delivery packages, etc.
(ALMA: Agile Lifecycle Management Anatomy)