what is the purpose of vCloud Director exactly

hi all,

i working on vmware virtualization for an year and half now, i want to plan learning vCloud, when i was watching videos, i came to know that it is like creating virtual datacenters for the vms created in vShpere or vCenter. My doubt is when we can create separate datacenters in vCenter itself then what is the use of vCloud Director Of course i saw lots of features of it, but what is the actual purpose of creating datacenters again from the vms of vcenter.

Please explain any one in simple i'll start working on it.

thanks in advance.