Looked up a quick write-up from serious eats.

"Why Sous-Vide?

Sous-vide precision cooking offers unparalleled control over the results of your steak, letting you very precisely cook the steak to the level of doneness that you prefer. No more guesswork to guarantee a medium-rare temperature. No poking with a thermometer, no cutting and peeking, no jabbing with your finger—just perfect results every single time.

It also increases the flexibility of your schedule. With traditional methods, your schedule is at the mercy of the steak. Once you start cooking it, it's a straight, nonstop shot until the finish line. Sous-vide steaks, on the other hand, can be held for several hours before finishing them by searing and serving, which means that your steaks are ready when you and your guests are.

Finally, sous-vide offers results that are not attainable by traditional methods. With standard high-heat cooking, you develop a temperature gradient within the meat. The very center may be perfectly medium-rare, but the steak will be increasingly more well done as you approach the exterior. With a sous-vide steak, your meat is evenly cooked from edge to edge.

Cooking steak sous-vide is a two-phase process. The first phase involves sealing the steak in a plastic bag and cooking it to the desired final temperature in your sous-vide device. The second phase is to sear the meat to develop color, flavor, and textural contrast on its surface, and to help render and soften its fat.

The temperature of the sous-vide bath during the initial cooking phase is what determines the final doneness of your steak."

Source: http://www.seriouseats.com/2015/06/food-lab-complete-guide-to-sous-vide-steak.html#sousvidewhy