Instant clone is a technology to create a powered on VM using as source another running VM. An instant clone VM shares memory and disk state with its source VM. Once it is powered on, the instant clone is a fully manageable independent vCenter Server object. The clones can be customized and have unique MACs, UUID. This makes the technology very appealing for use cases where large number of VMs need to be created in a short time from a controlled point in time - think about VDIs.
My use case was on-demand labs generated from the same lab template(s). A lab template is made of 3 to 6 VMs of different sizes running interdependent applications. Users login to a web app and then request one or more new labs from the available templates. The web app would start in the background lab provisioning for all the requests via vCenter Server.