Medically, it's most commonly used to supplement iodine in people who are deficient for some reason (usually because of insufficient dietary intake). It's also used to treat so-called thyroid storm in people with hyperthyroidism, since giving large amounts of iodine suppresses the thyroid gland.
The last purpose is to protect the thyroid from radiation - radioactive isotopes of iodine are sometimes used for medical imaging or as part of cancer treatments, and are also present in fallout from leaking reactors, nuclear weapons, etc. Since the thyroid stores iodine and uses it to make thyroid hormone, these radioactive isotopes will hang around in the body for a long time and can cause a lot of damage. By giving large amounts of iodine (in the form of potassium iodide), it saturates the thyroid with non-radioactive iodine, and since it has all it needs, the thyroid won't store the radioactive stuff. Since nothing else in the body uses iodine, the radioactive iodine will simply leave the body in waste products, minimizing the duration of exposure.
The last purpose is to protect the thyroid from radiation - radioactive isotopes of iodine are sometimes used for medical imaging or as part of cancer treatments, and are also present in fallout from leaking reactors, nuclear weapons, etc. Since the thyroid stores iodine and uses it to make thyroid hormone, these radioactive isotopes will hang around in the body for a long time and can cause a lot of damage. By giving large amounts of iodine (in the form of potassium iodide), it saturates the thyroid with non-radioactive iodine, and since it has all it needs, the thyroid won't store the radioactive stuff. Since nothing else in the body uses iodine, the radioactive iodine will simply leave the body in waste products, minimizing the duration of exposure.
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It is used to keep radioactive iodine from being absorbed into the thyroid gland, thus (hopefully) preventing cancer or other damage to that organ.