it's to control the smoothing between dynamic layers in dynamic programs. slider at bottom = full dynamics, at top none.
depending on how the program was made it might do nothing depending on the type of smoothing (there are loads), if it does do something it is very useful to turn off the dynamics and hear the program like that.
it was there originally to reduce artifacts if need be, it can be kept or removed by the program developer. in the old days everyone just left it in anyway, but now often its removed.
i use it often to check if an artifact is from the switching/smoothing or something else.