The body needs a circulatory system because we are complex multicellular organisms. A uni-cellular organism can happily live in a pond of the sea and the pond/sea will look after supply of O and removal of CO and other important molecules.
We are quite watery, in fact you could say that we carry our pond around with us. The job of the circulatory system is to keep things moving in our internal pond so that the mix of important molecules in the pond can be kept happy. If there is too much of a certain molecule then that may be bad (for instance K , CO ) and if there is too little of other important molecules then that too is bad (for instance O , glucose). Thus it is important to regulate and maintain safe levels of these important molecules.
In addition the function of the circulatory system depends on the function of the pumps (heart and legs), tubes (arteries, capillaries, veins and lympatics) and neither too much nor too little fluid in the system.
Part of the care of any patient, ill or well is to help maintain safe levels of important molecules and to try and keep the circulatory system happy.