Sometimes a large pharmaceutical manufacturer discontinues a medication. Often this happens because not enough patients are taking the drug, so it is unprofitable to keep mass-producing it. But what about the patients who still need that drug? Hundreds or even thousands of patients still may need that medication.
A compounding pharmacist can re-create that medication by compounding it, so even if only one person in the world still needs that medication, they can have it thanks to compounding!