How System Context / Use Case diagrams can help you define scope for your IT system
One thing I like about system context / use case diagrams is that they help me define and uncover scope for an IT system. To see what I mean, let's say I was called in to help understand or build an IT system that has a touchscreen device like this:
A preliminary system context model (with an underlying use case) would look like this:
Pretty simple. But not the whole context. Let's dig further.
For any information that a user reads in an IT system, there is an implicit create information use case that has to exist. There may also be an update information and delete information use case (think C.R.U.D.), but there is definitely a create information use case.
Now in this example, the shopper is not creating items to search for. There must be another actor who is doing that. The person responsible for this IT system says that an Admin person does that. The diagram expands to something like this:
We may wonder if each item is added individually, or if there is a process to automatically add items. Through interviews we find out there is an inventory system that transmits information to our IT system and adds items to it overnight. Now our system context looks like this:
In conclusion, by asking questions about the information our initial user ("Shopper") is searching for, we find our scope has expanded to include two new users/actors: an Admin and an Inventory System. The scope of our new IT system is now better understood.
P.S. Go back to the photograph and look at other items on that screen. There are use cases associated with them, too. Think about what information is associated with those use cases. Then think about how the system context / use case diagram would change to incorporate them.