Many reasons. Persons cough for various reasons. A cough can be associated with upper or lower respiratory infections, acid reflux, asthma (even without wheezing), post nasal drip, foreign body obstructions, lesions in the lungs such as cancer, and even damage to some nerves. There are many causes to a cough.
Reflex... Cough is a reflex designed to rid the airways of mucous containing bacteria, viruses, allergens, etc. By coughing, the body " cleans" the airways thus decreasing the possibility of infection. If secretions remain in the airways, they can become infected leading to acute bronchitis or pneumonia.