Over time, a masonry chimney can succumb to the elements. Years for fires and potential extreme weather take their toll. It may repairable, but at what cost? In some cases, you may not be able to get it repaired, but it is still structurally sound enough to not be a safety hazard. In such cases, a chimney liner is what you need. It will maintain adequate draft and safely carry smoke from your fire place to the outdoors.
In other instance, you may opt to add a different wood burned, such as a fireplace insert, which will require a proper fit between the exhaust port and the chimney. A properly fitted chimney liner, again, is just what you need. Remember, always follow the manufacturer's recommendation for optimum safety and efficiency.