Vm commit saves the state of the vm and commits it back into the base image. When you perform the commit, it requires a restart. Live commit preserves the state of the vm while the vm is running and does not require a restart. Each time a vm that was live committed is deployed, it comes up in the exact state that it was in. So if you performed a live commit on a vm that had had some apps open or a running process, it will start up in that exact same state. It’s like an instant on.
Facebook wants to save the state of running processes so they can observe the behavior of a payload and revisit it. Deleting the vm wipes this.