Actually, wolves both bark and howl. Researchers have identified at least 11 types of wolf call - the yelp, whimper, whine, whine-moan, moan, growl-moan, growl, snarl, woof, bark and howl.
With the exception of the howl, these are all short- to medium-range noises communicating intimate emotions, and are directed mainly at family members.
While the bark is used as a threat or protest - for instance, when a human or other large predator approaches the den - howling is a relatively low-frequency, elongated call designed to carry over large distances. On the open tundra, wolves can hear a howl from 11km away.
Howling probably has four main purposes. It helps pack members to stay in touch and coordinate movements across their enormous home ranges (normally 200-600 square kilometres, but occasionally much larger); it enables groups to advertise their presence and claim on a territory, avoiding unnecessary encounters with rivals; it may help lone wolves to locate potential mates; and it apparently strengthens social bonds within the pack when performed in chorus.
Young wolves can travel huge distances once they leave home. The record is held by a Minnesota female, who covered 4,117km in search of company. A good howl might have helped her.