Premature infants have difficulty maintaining their body temperature, regulating their breathing, and feeding properly. Although they may look developed on the outside, their internal organs may still need assistance. In particular, they usually need help with breathing (because their lungs are immature) and help with developing and growing their brain. Neonatal units help babies perform these functions by placing them in an incubator to maintain temperature; using a ventilator or respirator to help them breathe; and/or potentially placing a small tube through their nose or mouth directly into their stomach or an intravenous (IV) line directly into their bloodstream to give them food until they are able to accept food into their stomach. 

(Brian Hall, Wikicommons, Public Domain)

Reference: AbbVie Corporation (Ed.). (2015). Preemie Parent Handbook (4th ed.). Canada: AbbVie Corportation.