A stove will only perform at its optimum output and efficiency when connected to an appropriately sized and sealed flue system. Without this, a sufficient draw cannot be created to actually power and control the heating appliance below.
By lining an existing chimney with Flexible Flue Liner, you ensure the installation is safely sealed from stove to terminal, creates the sufficient draw to extract and exhaust the by-products of combustion (rather than pouring these into your living room) and complies with all building regulations and approval schemes.