If you perform using a USB or MIDI keyboard controller, you can play and control MainStage patches with software instruments using your controller. You can assign faders, knobs, buttons, and other controls on the keyboard controller to screen controls in your concert, and then map those screen controls to parameters in your patches. You can choose exactly the parameters you want to have at your fingertips for each patch and access them from your controller as you perform.
You can use MainStage with other MIDI controllers, including sustain pedals, expression pedals, foot switches, MIDI guitars, and wind controllers that send standard MIDI messages. You can also control external hardware synthesizers, ReWire applications, and other virtual instruments using external instrument channel strips.