IV Fluid Therapy is important for many reasons but the most common reason for a Veterinarian to recommend IV fluid therapy would be in to correct dehydration in your pet.
Your pet can become dehydrated from many different causes, ranging from not drinking/eating enough, to prolonged vomiting/diarrhea resulting in fluid loss. Another reason for IV fluids is kidney failure which results in the body losing fluid due to the kidneys not being able to adequately concentrate the urine.
Another important reason for IV fluids is the administration of medications, which can range from additional pain medication, to antibiotics, to life saving treatments.