In the case of diabetes, when we give insulin, it is a replacement therapy, as we give injections to replace or possibly supplement the natural insulin the body is lacking. Addison's disease, Hypothyroidism, and Pancreatic insufficiency are also treated by replacement therapy--providing the hormones or enzymes the body needs to function properly from outside sources.
Fluids given to combat dehydration, whether they are given orally or subcutaneously, are a form of replacement therapy, because it is a way to replace the lost and necessary fluids and electrolytes. Prescribed vitamin supplements or injections are another form of replacement therapy. Blood transfusions can also be thought of as replacement therapy.