I personally use a shoulder rest because I have a long neck and I need to raise my shoulder to keep the violin in place (which I even had a problem with when I didn’t have a shoulder rest that was high enough) and it cause me a lot of problems with muscle tension. I also find that using a shoulder rest changes the sound of the violin, I don’t know how much of a difference it is to the listener but with it so close to me, without the shoulder rest the sound seems to be dampened and because my shoulder rest barely has anything touching my violin, it seems to resonate better compared to my old shoulder rest.. Could just be my imagination though haha.
I usually try to get my students to use a shoulder rest unless they have a very short neck because it allows the player to be more relaxed.
My violin literally just sits in place without me trying to hold it up with my hand and grab it between my neck and shoulder. Just having everything sitting the right way keeps it there.