Why should you need iron supplement if your body can recycle iron?

Only if iron is low. Iron supplementation is not needed unless there is iron loss through bleeding or inability to absorb iron from food or water. The only way one can lose iron is through bleeding, either from excessive menses, or bleeding from the bowel. Bleeding from other areas is usually very noticeable.
Not 100% recycled. Some iron is lost because the body cannot recycle and reuse 100% of its iron. Therefore, a dietary source of iron is needed. However, taking excess iron (more than the body needs) eventually becomes harmful. So, one should discuss with his or her doctor to see how much iron supplementation is needed, if any. Many people get enough iron from their diet, but some do need iron supplements.