technically it's not "boiling" the meat because the meat is not in contact with the water but instead in a sealed bag. some vacuum pack them, I use zip lock bags. the meat only cooks in its own juices, marinade or seasonings you put in the bag. since it's sealed little moisture is lost.
the water is only used to provide the exact temperature control. you can use vegetables oil, dish soap or even ATF as long as you don't let it contact the food. water is used for the obvious reason of availability, price, safety and ease of cleanup. one can in theory do this dry like in an oven but they normally have horrible temperature swings and bad accuracy at the temps we need for an extended cook