index value is the offset provided to the base address . the basic kind of indexed addressing as explained in Carl hamacher book is the X(Ri) which is used in the code snippet.
X is a fixed offset that is R0 + 4 bytes , this offset cant be changed
Whereas the R0 is base address which can be changed either at beginning of program or as in the loop shown in the snippet by adding #16 to Ro .
The other mode you are talking about is the register offset variation which provides (Ri , Rj) option to even change the offset . so we dont need 3 instructions for addition we can create one ADD instruction like
L1 : MOV #4,R4 ; set offset of R4
then increase the offset counter R4
Now Branch to L1 with a counter to set the no of times to add 4 to the original offset
This type of code is like working with 2d matrix with (Ri , Rj) being equivalent to rows and columns.