performance tuning - What are the main differences between RawArray and PackedArray?

What I know:

• 
  

  PackedArray is documented (to some extent) in "Developer Utilities Package", RawArray is just mentioned in Raster3D and ImageApply,

  
  


• 
  

  RawArray supports Byte, SignedInteger8/16/32, Bit16, Real32, Real and Complex128 as entries, while PackedArray only Integer, Real and Complex(128),

  
  


• 
  

  PackedArray displays as a list but RawArray as (somewhat of) a skeleton.

  
  

What I do not understand is why there are two of them. Why not just stick with the one that supports more entry types and display it as skeleton if and only if it is too long?

Also, how does ByteArray fit into the picture?

Edit: You can create a PackedArray using:

ap = Developer`ToPackedArray[{1, 2, 3}]
(* {1,2,3} *)
Developer`PackedArrayQ[ap]
(* True *)
Developer`PackedArrayForm[ap]
(* PackedArray[Integer, <3>] *)

(Note that there is in fact no PackedArray command, the head of the last output expression is a string.)

You can create a RawArray using:

ar = Developer`AllocateRawArray["Byte", {2, 2}]
(* RawArray[Byte, <2,2>] *)
Developer`RawArrayQ[ar]
(* True *)
Developer`RawArrayType[ar]
(* Byte *)

(ditto about RawArray).

You can create a ByteArray directly:

ab = ByteArray[{1,2,3}]
(* ByteArray[<3>] *)
ByteArrayQ[ab]
(* True *)

All the cross-tests (e.g., RawArrayQ done on ab) return False.