You bought a better chair.
Your back still aches.

If your feet do not sit flat on the floor when you are seated, your lower back is taking the difference. That is the whole case for one. If your feet already rest flat, you do not need it.

Yes. It needs a flat surface behind it to brace against, so it works against a headboard, a wall or the back of a firm sofa. It will slide on a very soft, deep couch.

It folds flat and stands against a wall or slides under most sofas. That was the point of choosing a folding one over a fixed ottoman.