Most stones *will* pass with normal bowel action if your bowel isn't in bad shape. Sometimes not all come out that way though. One thing the enema can do is wash out the stragglers that are waiting for the next day's bowel movement. Another thing it can do is remove them from folds in the bowel where they can get stuck. Adding lemon juice to the enema can make it antiseptic which I kind of like.
coffee enemas the day after, or even two days later, can cause the GB to release even more stones. Same for colonic.