In a heterogeneous vSphere environment, you might end up having hosts in a cluster with same make CPUs but with uncommon feature sets. For vMotion to work, the underlying CPU features on the cluster hosts should be identical. With EVC, you can now present a common CPU feature set across the hosts to the VMs.
EVC enables a common baseline across a heterogeneous set of hosts, so it is quite possible that most of the already running VMs are using CPU feature sets that might become unavailable after setting the EVC baseline. Hence, it is recommended to either power off the VMs or evacuate the cluster (by migrating (VMotion) the VMs to another cluster or a set of standby hosts), though you will have to power off the VMs before they are moved back into the EVC cluster running a lower baseline. If you don't have enough hosts in your environment to move the VMs out temporarily, create a new EVC cluster with the required baseline and then start moving empty hosts...