McAfee ePO server is the central software repository for all McAfee product installations, updates, and other content. The modular design of ePolicy Orchestrator allows new products to be added as extensions. This includes new or updated versions of McAfee and McAfee-compatible solutions from the Security Innovation Alliance. Packages (VSE, ENS, CMA so on) are components that are checked in to the master repository, and then deployed to client systems.
You do need a license key for ePO; your license key will only allow access / deployment of purchase products (VSE, ENS, CMA so on). So, ePO by itself doesn't provide any protection; rather, ePO is used to manage products that you have purchased via your license key.