What is the purpose behind the World Trees in World of Warcraft

The original World Tree (in Mount Hyjal) grew on top of the Well of Eternity, and forms a sort of bond that keeps the energies of the world together and prevents the Well from becoming unstable as well. Although it was damaged or destroyed as part of Archimonde's attack, a new sprout was seen growing from the trunk.

Archdruid Fandral Staghelm attempted to recreate the World Tree by planting a sprout, resulting in Teldrassil. However, the tree is not healthy, and may even be dying. In Northrend, another attempt was made to recreate a World Tree, Vordrassil. However, this one was tainted by the power of Yogg-Saron in the ground.