Sleeping bag liners perform a number of useful functions. Firstly they keep your sleeping bag clean and stop the need to wash it which reduces its performance. Secondly they can add 1-2 degrees onto the warmth rating of a sleeping bag by adding an extra layer of material and air. Thirdly they can be used instead of a sleeping bag in warmer climates making them perfect for backpacking in hot countries.
Sleeping bag liners also come in a variety of materials but usually cotton or silk. Cotton is much cheaper then silk but silk is much lighter and packs down to a ridiculously small size! Some of our liners also have an insect repellent and antibacterial agent built in.