Liners serve three main purposes: to protect the home from heat to combustibles; protect the masonry; and provide a properly-sized flue to provide maximum performance of your fireplace or heating appliance.
Fireplace chimneys may need to have a stainless steel liner installed if the existing flue tiles are badly deteriorated or have been damaged by a chimney fire. Older homes that were built without flue liners are not safe to use and require relining.
Chimney flues that are used as an exhaust for gas furnaces and / or water heaters should also be relined. Most existing masonry flues are oversized for today's gas appliances. That creates an acidic condensation inside the chimney. The condensation can damage the inside of your chimney as well as the brickwork.