Shortly…
If we use some FX we can use it by GUI (if exist), or by typical Renoise sliders in channel’s chain.
Mostly of the effects has own GUI, but in GUI sometimes hasn’t represent all parameters of FX, that can be modified by user. In the slider view of FX we have assigned ALL available parameters to sliders.
Simple Squeeze has only one parameter on GUI in slieder view we can see a few more parameters to realtime control! Simple Squeeze is only an example. Most of FX hasn’t assigned all available parameters to GUI, but we can use it by sliders 