A liner is by no means essential, but many parents choose one because it makes dealing with a pooey nappy so much easier. Liners are designed to catch the poo while letting the wee pass through into the nappy. Disposable paper liners can be thrown in the bin or flushed down the loo, whereas washable fleece liners are rinsed off and washed with nappies. Fleece liners have the extra benefit of allowing fluids to wick through to the nappy, creating a "stay dry" layer against baby's bottom - particularly helpful at night. They also tend to stay in place better on a wriggly baby.