Depth helps You in case of 3D drawings or models.
After changing of an slice position ASCAN will change active depth of an corresponding view.
For example:
If You draw on the View 1 (TOP) and active depth=0 then Z coordinate of elements have value 0.
Then if You move red slice to Z=1 and then You will draw, elements will have Z coordinate = 1.
In case of 3D drawings Depth lets You work more efficent and avoid many misstaeks.
See also:
Depth ON/OFF
Set Depth