I'm probably missing something, but here's an example.
I have an audio plugin that simulates an electric piano. My PluginProcessor is the parent of an Editor and the EP simulator. Inside my EP sim I have a phase shifter and a flanger.
The PluginProcessor has an AudioProcessorParameterFloat called volume that is controlled by a ParameterSlider or similar, so that when I move the volume slider in the editor, it changes the parameter value for the volume inside the EP sim. That makes sense to me.
But when I have a parameterInt for choosing between the phaser or the flanger, when that value changes, I need to run a function inside the EP that makes some changes. So changing the int value of isn't enough. When the user changes the switch from phaser to flanger, some code needs to change a bunch of things.
See what I mean?
Maybe I'm missing something terribly obvious.