Proteins are used for many structural functions such as in the actin and myosin in muscle or as a part of the cytoskeleton scaffolding that maintains cell shape . what other main function do proteins serve?

Proteins are made of amino acids. In the cell, those amino acids are strung together as new proteins that your body can use to get reactions going. Carbohydrates also need to be balanced with fats and protein to maintaina healthy weight.
Basically, they control the rate of reactions and regulate cellular activity.