It first came into importance during the Second War when the tides of darkness descended on Lordaeron and its member nations. After Gul'dan raised the Tomb of Sargeras from the seafloor and the Black Tooth Grin clan defeated Gul'dan's followers near the Tomb, the clan's chieftains received order to sail to Stormwind, then travel to the Blackrock Spire. However the orcs' retreat was blocked by the Kul Tiran armada, which intercepted the Horde navy near Crestfall. Both fleets received aerial backups but the Horde's red dragons were kept at bay by the Alliance's gryphon riders. As opposed to the events of Warcraft II, current lore does not mention a naval base near Crestfall, also the retreat was triggered by the Alliance's successful defense of Lordaeron City, instead of the destruction of Alterac.
In Warcraft II: Tides of Darkness
This section concerns content exclusive to Warcraft II: Tides of Darkness.When Alterac was destroyed, the orcish armies in the north staged a massive retreat. Kul Tiras' admiral, Daelin Proudmoore sent word that gnomish submarines had located the Horde's main naval base near Crestfall. Proudmoore believed that the Orcs planned to launch the remainder of their armada and retreat to the mainland of Azeroth. Lord Lothar ordered the base at Crestfall to be destroyed before the fleet could escape. This was known as the Battle at Crestfall.(WoWMan 152)(W2Man #?) 
This section concerns content exclusive to the Warcraft RPG, and thus unlikely to be canon.Crestfall houses a sizable community of boaters and fishermen. These people do not like strangers, some visitors can be nearly shot when approaching a town at night on this island. The riflemen are constantly on guard from the naga that have been plaguing them recently. Rumors abound that a large naga force, under a warlord called Zethresh, hides near Crestfall’s west shore. Zethresh reputedly seeks to cleanse the ocean and the isles of human encroachment.(LoC 101)