Why Get a Liner?
If you burn wood with an unlined chimney or a damaged liner your creating a fire hazard. Chimney fires burn hot enough that framing around the chimney can catch fire right through a layer of brick. The liner is your homes essential layer of fire protection.
A properly-sized flue creates the ideal draft for your furnace, water heater, fireplace or stove. Over-sized flues (often the case when the flue is unlined) tend to provide too weak a draft for modern appliances. A chimney liner will help improve the draft, which in turn improves the operation of the appliance.
Easier, more effective cleaning
Ask any chimney sweep on the planet which is easier to clean: an unlined chimney, or one with a properly sized liner. The lined chimney is always easier to clean, because the improved draft means less condensation of smoke and gasses (less soot and creosote buildup) and the flue is an even, consistent size with an even surface that the brushes can clean effectively.
Aside from chimney fires, earthquakes, falling trees and the like, the number one killer of chimneys is water. Especially in northern climates, freeze-thaw cycles slowly break chimneys apart. You’ve probably seen chimneys with missing bricks, or chimneys leaning to one side due to years of wind-driven rain soaking into the masonry. But water damage can happen on the inside, too. Water vapor is one of the components in flue gasses. In the case of chimneys that are used to vent gas-fired appliances in particular, the inside is sometimes literally soaking wet, just from condensing water vapor.
A chimney liner acts as a barrier against interior damage from condensation, protecting the inside of the chimney from gradual decay cause by water penetration.