Although attorneys manage the procedural aspects of a case, to the great relief of their clients, this is not the main service the attorney offers. The key service is issue spotting and problem resolution long before the consular interview. Mistakes at the Embassy/Consulate interview can cost months and even years of delays. Not all lawyers are the same in this regard, either. Lawyers can vary greatly in skill and experience. The smarter and more experienced the attorney, the more likely he is to spot issues and resolve them in advance, and the lower the chance of a painful delay later.
We can't emphasize enough how important it is to identify possible problems and start resolving them long before the petition is submitted. We have found that most of our own cases that aren't approved on the day of the interview are "Embassy Only" cases we picked up after the client had filed his own I-129F. We're still somewhat puzzled by this because we scrutinize every such case very carefully before we accept it and attack all the problems we see. But we suppose that we just don't get to know the clients and fiancée as well due to the shorter time the case is in our office, or perhaps because there are problems in the original petition that continue to haunt the case.
We rarely accept such cases now. When we "live" with the client from the beginning of the visa process until the end we are able to achieve a 95+% rate of first day approval of the K-1 or K-3 visa.