What are "decorators" and how are they used?

A good use case of $provide.decorator is when you need to do minor "tweak" on some third-party/upstream service, on which your module depends, while leaving the service intact (because you are not the owner/maintainer of the service). Here is a demonstration on plunkr.

Decorators allow us to separate out cross-cutting concerns and allow services to preserve the single-responsibility-principle without worrying about "infrastructure" code.

Practical uses of decorators:

• Caching: if we have a service which makes potentially expensive HTTP calls, we can wrap the service in a caching decorator which checks local storage before making the external call.
• Debugging/Tracing: have a switch depending on your development/production configuration which decorates your services with debugging or tracing wrappers.
• Throttling : wrap frequently triggered calls in a debouncing wrapper. Allows us to easily interact with rate-limited services, for example.

In all these cases, we limit the code in the service to its main responsibility.