These controllers are used by specifying the number, above, and also giving a value. The standard or default value for pan, for instance, would be 64, right in the middle of 0 and 127, centered between both speakers. The default for modulation would be 0, or none at all. Now the defaults are what you get if you do nothing. But of course you can change them to whatever you want, even and often particularly as the MIDI is playing.
People sometimes like to pan back and forth between the speakers for effect.