Adding a liner makes your chair removable. If you add a liner your foam is incased in a poly/cotton blend material with its own zipper, for easy refilling. The cover is then put over this liner. If you buy the liner you can remove the cover and wash it or replace it with ease. If you do not buy a liner your cover will come filled with the foam directly inside it and removing the foam to wash or replace the cover will be a messy task. We strongly recommend purchasing the liner with your sac.