There is a lot of discussion about whether or not you should add butter, oil, or other fats to your sous vide bag. Here's a look at some of the issues so you'll know how to maximize your flavor. The butter argument depends on what type of protein you are cooking, so I'll address meat and fish separately.
When it comes to simple but flavorful food nothing beats a perfectly cooked steak with melted butter on it. Because of this, many people put butter into their sous vide bags to "flavor" the meat. However, there are a few things at play here, especially for red meat like beef or lamb, but also to a lesser extent chicken and pork.
The main issue is that the meat doesn't actually absorb any flavor from the butter. In fact, the butter can actually pull some fat-soluable compounds out of the meat, reducing the flavor slightly. If you end up using all of the juices in the sous vide bag for the main dish, this isn't too big of a deal, but you are not adding any flavor.
In most cases, the best way to add butter to a sous vide steak or chicken is after it is done cooking. Sous vide the meat, then apply the butter at the end of the searing process or when you are making a pan sauce. This will give you all the butter flavor you are looking for, but it will ensure you are not diluting any flavor from the meat itself.
Some people do recommend putting fat in your bag to help things keep their shape, to prevent sticking, and to remove air packets. These are all good reasons to add fat, but it is not adding flavor to the meal.
Fish are a different case than red meat when it comes to butter and oil. Because most fish have very little fat, there are usually no fat-soluable compounds that can get pulled out. Many people also think fish actually does absorb some of the flavor from the butter or oil.
Having fat in the bag also helps keep the fish from sticking together and maintain their shape. Some fish like salmon also release a white substance during cooking and the fat will dilute that and prevent it from sticking to the fish itself.
I think the benefits of adding fat to sous vide fish are worth it and it is something I generally do.