I think it just depends on how you look at that. True, the BBB can't actually make a business do anything that a complaint is being directed towards, but they have no choice but to record the complaint and keep it on file regardless of whether an agreement is eventually reached between the business and the complaining party.
That complaint and the eventual result is then kept on record, can be compared with any similar occurences in the future to determine if a pattern is being established, and any patterns that do occur can be used as evidence in further legal actions if necessary by customers with similar complaints. The fact that the initial complaint and the eventual results will be kept on file is sometimes enough of a factor in itself to influence what the business will be willing to do.
If nothing else, the BBB is an initial no-cost alternative to settling disputes with businesses who are members of the BBB (and those who aren't), and who would prefer to maintain their image as being reputable. The BBB route isn't perfect by any means, but there's also something to be said for trying to work things out through an intermediary since more often than not, people tend to automatically go on the defensive when dealing with each other in person.
But again, the BBB is just one option. Whether or not you think you may be able to work things out with the owner in person just depends on the owner and how you make your approach.