The lower temperatures cooking sous vide helps with retaining moisture in meat. While the myosin contracts in the 104-122 F temperature range, actin denatures at a much higher range of 150-163 F. Braising takes the meat up past the actin temperatures, but with sous vide you can keep it below.
(Note some online sous vide recipes say to use a higher temperature, so look for ones that keep the temperature below 150.)
A good explanation with pictures is here: Heat and Its Effects on Muscle Fibers in Meat
