This function is used to move active layers to a new document. It is a good way to save out a textured layer to use in another document.  

Accessed from the Layer Menu, the option Split puts the active layers into a new document, removing them from the original document.

If you hold down ⌥ (option key) on the keyboard whilst in the Layer Menu, you will notice the Split option changes to Split a Copy. This puts the active layers into a new document, leaving the original document in tact.