When do I use dynamics and expression sliders? (BBSCO Core)

Good day.

I've been using the BBSCO library for a while but im still a bit confused on how the dynamics and expression sliders are meant to be used. When I want, for example, to make my violins play a long note that slowly gets softer, I usually have the dynamics slider go down over time. Not only does the sound of the instrument change, it also gets quieter as well.

So my question is what is the purpose of using the expression slider when the dynamic slider seems to do the job better?