From the vCMP Admin Guide:
When you move a vCMP guest to the Provisioned state, the system allocates resources (CPU cores, physical memory, and so on) to that guest. The guest also creates virtual disks for the guest and installs the selected ISO image on them. A guest does no run while in Provisioned state.
You may have change controls that you have to abide by for bringing things online, or perhaps you are preparing for future work. I guess there could be many reasons why you would want to prep an instance an not actually power it on.
You can re-attach an existing disk to a new instance:
http://support.f5.com/kb/en-us/products/big-ip_ltm/manuals/product/vcmp-admin-11-2-0/8.html
Regards,
Eric