What is the fixation on long cook times? Those are only needed to get to the middle.
There are 3 main benefits to sous vide.
1. Achieving the same temperature through out the finished product. In traditional cooking, to get the center of a steak/roast to 131 (medium rare) there will be a gradient of temperatures through out the finished product. Just below the surface will be well done, then medium till finally medium rare. With Sous vide, since the item is surrounded by a stabilized water temperature the result is the same temperature (131) through out the roast/steak. A quick sear for the maillard reaction allows end to end medium rare will little to no gray areas.
2. There are certain cuts of meat (with short ribs being a prime example) that are very tough. With traditional cooking methods, long cooking time would dry out the meat. Due to the fact the the meat is submerged in a vacuum sealed bag in water this prevents the meat from drying out. Sous vide can transform these cuts of meat through collagen breakdown to soft, fantastic dishes. You wouldn't sous vide a steak for 72 hours no matter how large it is. But short ribs are delicious after a 48 hour or 72 hour cook. It all depends on the cut.
3. In certain settings, it's convenient. Making a BBQ for 50 people and don't want to sit grilling while everyone is having fun? Sous vide the burgers and steaks a few hours before and when everyone shows up, throw it on a smoking hot grill to get that flavor and sear. Chef at a restaurant and feeling rushed during dinner? Throw a bunch of entrees in the sous vide and when the order comes in all it needs is a sear.
There are other food items that you can sous vide as well (eggs, duck, vegetables even desserts) but the reasons why you would sous vide would usually fall under one of the reasons or a combination of the reasons above.