Normal j-boxes are typically used for new construction. Those are the plastic boxes behind your wall outlets and switches. The problem with using them is when you have a lot of wires going to the same place it is very cramped and makes it difficult to get them all in there. I think it is very problematic for things like RG6 coax cable which is very thick and requires a minimum bend radius of two inches - just try to cram two or three of them into a jbox; you won't be able to do it without crushing them and risking breaking the center conductor.
A retro ring is used after construction because once the drywall is up you can't get in there and nail a j-box to the stud. An advantage to a retro ring is that it is not an enclosed box and there is plenty of space in the wall cavity for multiple wires without having to bend them excessively.
As an example, I have a double gang wall plate in my living room and the builders ran 9 16/2 speaker wires, 4 16/4 speaker wires, 2 RG6 coax, and 2 CAT5 and jammed them all into the double gang j-box. Not all of the wires are used and it was a mess. With the retro ring, the unused wires are now hanging freely in the wall cavity and only the wires that are used are attached to the wall plate.