i will take the exemple with an actor character
(set world location) = to set something to a destination for exemple you want to set the player to the location x100,y100,z100 on press B then when the key will be pressed the whole actor will be teleported to the location
(add world offset) = in my definition is to add location offset of something exemple the camera of the player, if you set the camera of the player x0,y0,z50 then the camera will be more in air cause you added 50 to the Z axis
if you want to teleport something use set world location if you want to offset something use add world offset 