As your environment grows, vCenter becomes a critical management tool, which raises the requirement for it to be highly available.
When vCenter is not available, the environment doesn't go down because all the core functionality is still there. All VMs will still be running; HA and FT will continue to function.
Note
Also, it's important that all hosts and guests continue to function only for 14 days. After this grace period, critical functionality, such as powering on ESXi hosts or starting VMs, will cease to operate.
What will be lost is the ability to perform a lot of administrative tasks that are not available without vCenter such as:
vMotion and Storage vMotion
HA and FT configuration
DRS manual tasks
DRS recommendation generation
Deploying VMs from a template
Resource availability information updates for HA
Issuing new licenses to hosts