evCushy Pods are made to be used with no liners but the extra mattress works well when you want to protect pod from spillages of milk etc. Using Liner you don't need to wash the whole nest. The extra mattress is also an additional insulation layer so if you plan to move your nest around and keep it on the floor we would suggest buying it. The liners also fit Moses Basket and prams. Its the alternative use of the liners which makes them very handy.