Why do Z-Wave sensors have multiple child devices doing the same thing?

[SmartyPants]

As the title states, I was wondering why Z-Wave sensors have multiple child devices doing the same thing?
For example, all of our contact sensors and/or PIRs have something similar:

Home Security Notification
Binary Notification

Both these are effectively the same thing in that they turn on or off to provide notification, so why not just have the binary switch?
I understand that the binary switch notification is a simple 0/1 (on/off) type, and the other has some extra functionality, but how is this extra functionality accessible?

Thanks