The answer is very simple and short
Imagine the following scenario, you create a SmartCloud Application Collaborative Lifecycle Management Service Instance or a SmartCloud Application Workload Service Instance and you start working with it, all data are stored in the virtual machine.
Now, what will happen if the VM crashes or if you have to do a release to a higher version! You will LOOSE your data, for the crash it is obvious as your VM crashes you will loose your data but for the release to a higher version you will have to kill your current instance and thus destroy your data and re-provision a new instance.
So, if you use a persistent storage all data will be saved on this disk and the lifecycle of this disk is independent of the lifecycle of the VM. So, you can re-attach this disk to another VM (a new one or a new one with a higher release) and thus keep all your data.
I remind you also that persistent disk are RAID protected and you can easily clone or backup them.