What do you mean by "Primary machine"? The Web service only supplies the AccessPoint static content to end-users. In addition to that, and on condition that you have a cluster license and multiple QVS machines, the web server will forward users to one of the configured cluster node QVS machines based on the selected load balancing strategy. That's the most simple QVS clustering setup.
But as soon as you start clustering machines, there is a primary machine in name only because that's the one you configure first in QMC. Not because it has any special function or because it'll do more work than the others.