The operations of the deployment server are governed by serverclass.conf. When you populate this file to tell the deployment server about classes of clients, and what apps they should receive, that's the first part of the process. Then, when you restart Splunk with a serverclass.conf present and enabled, that's basically the same as "splunk enable deploy-server". I believe that the latter is provided so that you can enable/disable the functionality on a temporary basis, say, for triaging a problem, without restarting Splunk.
Oh! I see. I thought even after I provide serverclass.conf file with no content (only [global] stanza), I need to issue the command to actually enable the deployment server. What is the minimal configuration in serverclass.conf so splunk will be enabled as deployment server? * it is mostly to see if deployment clients are checked in before actually deploying any apps.