World Displacement works very much like World Position Offset, but it uses Tessellation vertices rather than the base vertices of the mesh. In order for this to be enabled, the Tessellation property on the Material must be set to something other than None.
When using World Displacement to expand your object beyond its original bounds, keep in mind that the renderer still uses those original bounds. This means that you may see culling and shadowing errors. To address these issues, you can edit the mesh's Scale Bounds property, although your edits could result in a performance drain, potentially leading to other errors (such as improper shadowing).