The cellular plasma membrane does not simply form a "sack" in which to keep all the cytoplasm and other cellular organelles. The plasma membrane is a very important structure which functions to serve as a boundary between the cell and its environment and it is involved in the absorption and secretion of materials. It is selectively permeable allowing only small molecules to pass through and other substances like ions can be transported across the membrane by active transport.
The protein molecules in the plasma membrane also vary in function. They provide structural support and contain channels allowing transport of small molecules through the membrane. They also act as carriers which actively 'pump' molecules across the membrane and act as receptors for hormones arriving at the outer surface of the membrane. The proteins can serves as enzymes catalysing biochemical reactions in and on the membrane and can also serve as antigenic markers which identify the cell's blood or tissue type.