Fleece liners wick moisture away from your baby's bottom, helping them to feel drier, longer. They come into their own during the tricky weaning phase, making disposing of sludgy poo easier (too much information?!) Poo tends to fall off these liners much easier than anything else. Not an essential piece of kit but definitely useful.
During and after weaning, you will need to dispose of poo in the toilet, these liners help with that process. They also help protect nappies from stains.