They make your coughs more productive, thereby less frequent. Instead of a non-productive, dry, hacking cough (which is a waste of energy), you hock some good loogies and get the job done. Medically speaking, you are moving phlegm from the respiratory tract to the digestive tract (if swallowed) or out of the system ( if hocked or blown into a tissue.)
Just call me Dr. Mom.
Peace,
Amy