Liners are generally used for a couple of purposes.  With disposable liners you can catch your baby's solid waste and flush the liner in the toilet, making for an easy cleanup.  Fleece liners and "Stay Dry" liners wick moisture away from your baby's skin and pull it into the diaper below, much like a disposable diaper.  Silk liners are the only natural fiber that will wick moisture away from your baby's bottom and they also have protective properties to help with diaper rash.