Why is vCenter Server on its own server?

Hi, so this probably has an obvious answer, but given that I am still learning VMWare, and network administration in general, it isn't apparent to me.

I am setting up some virtual servers with Workstation 10 to practice on a vSphere environment. The topology I am going off of has the following:

2x ESXi Hosts

1x Microsoft Server with vClient

1x NAS

1x vCenter Server

I don't understand why vCenter Server has to be on its own server. Couldn't it be omitted and software like vClient just be used to maintain the data center? In other words, vCenter Server acts as an intermediary between the ESXi hosts and the vClient. But why not have the vClient and ESXi hosts communicate directly with MS Server with vClient essentially replacing the vCenter Server?

I guess one answer is just that that is how it was designed, but I am sure there is a rational reason.

Thanks in advance for the input and clarification!