roach backs can be from birth or from an injury.
If from birth, they often don't cause issues other than with saddle fit or with balance.
If from injury they may be the sight of pain, or may cause secondary pain as teh horse compensates unnaturally.
Types of issues that can cause a roach back; back issue, internal issues (usually also shows symptoms of colic), hind end lameness, in particular pelvis or stifle.
Interestingly I have a horse here that is developing a roach back due to unknown reasons. We are assuming pain, but cannot pinpoint. I do know that if I ride her she is not bad, but anyone else rides here and she is miserable...assuming because they weigh more. Everytime the chiro comes out she has something different "out" or sore...although her back itself doesn't show pain. It has been interesting to watch how her muscling and such has changed how her back roaches.
Treatment would depend on the cause, but it is possible that her horse is not in pain and that it is just the shape of her spine. yes, this would cause her horse to move differently and carry itself differently, but doesn't not necissarily mean pain.