VCHA is a feature that uses a three-node cluster to protect the vCenter Server from hardware, operating system, or application failures. The three nodes are referred to as active, passive, and witness. VCHA only supports VCSA deployments, not vCenter on Windows, and both embedded and external PSCs are supported. It's important to note that if used with external PSCs, VCHA is not protecting the PSCs—only the vCenter Server itself. Load balanced PSCs would be needed to provide high availability to external PSCs. Keep in mind that it likely doesn't make sense to use vCenter HA if you're not also using load-balanced PSCs, since the idea is to create a highly available management plane.
VCHA is useful when you want to increase vCenter's uptime and you don't necessarily want to only rely on vSphere...