Coughing is the body's way of removing the mucous from the lungs. A cough suppressant stops the urge to cough, but does nothing to stop the drainage of mucous into the lungs, leading to an unpleasant build up. Taking a cough suppressant at night may lead to a restful sleep, but in the morning you'll be rewarded with a chest tight with a thick mucous build up, leading to an extended bout of early morning dry coughing. This loosens up the congestion, allowing you to cough it up. Any time you have a dry, hacking cough that is non-productive, use an expectorant.
Runny noses are another bane of cold sufferers.
Once again, they too serve a purpose. This sudden increase of nasal secretions is the body's method of flushing out the nasal and sinus cavities. Nasal secretions routinely transfer cold and flu viruses, but stopping the runny nose isn't the answer. Your body is getting rid of the virus in this manner, by using an antihistamine to stop the flow, you're preventing your body from naturally ridding itself of the virus. Nasal and chest congestion are usually treated with decongestants.