What is the purpose of underwire bra?

Underwire in a bra will provide shape, not support. Once the fabric used in making the cups has stretched out (typically 6 months according to the manufacturers) the underwire will allow the breast tissue to sag over.

Underwire has many disadvantages. Underwires wear against the fabric and shorten the life of the bra. They are dependent on the fabric that is used in the cups to provide the support. They dig into the breast tissue and can cut off the circulation of the lymph nodes, which is the system that drains toxins out of breast tissue.