What is the purpose of global variables within a Design Study?

I recently tried to create a design study, and linked a few variables in my design study to global variables within my model. These global variables were used primarily to change dimensions within sketches. However, when I ran the Design Study, the outputs of my simulation did not change. Upon further inspection, the sketches/model was not changing. I thought that perhaps the model wasnt reloading between each pass, but I found out that the global variables weren`t even updating (by watching the "Equations" tab in my model design tree. Did I miss a crucial step, or do global variables serve a separate purpose that I am not aware of? I realize that there is a specific variable setting for "Model Dimension" that I ended up using, but it seems pointless to be able to link a parameter to a global variable, if the global variable doesn't update between each pass/step. Where am I going wrong here?

SolidworksSimulation