What it is in a nutshell:
Restaurants use a water bath (in a vacuum bag) at a set 'lower' temp then pull whatever (usually meat) it is and finish it on a grill or hot plate to customer specifications. You can have it in the bath for a long period of time at the low temp (I used 132*) without it ever being overdone. It just sits in its own juice at 'rare' for hours. Saves a lot of time. The temp is usually controlled and held within a degree. You can't hold it too low or bacteria will start to grow.
While I have done it at home it is really only useful if you are prepping a bunch of, say burgers or steaks or fish for a larger group. You set it up hours in advance then it takes a very short amount of time to finish.
It kind of became a 'thing' but a lot of people decide it is not worth the effort unless they are feeding a large crowd.
I should add it works like slow cooking in that it breaks down the collagen in the meat and can (should) make it very tender. You can put spices in the bag with the meat but if you are finishing it on a grill it works good to spice it just before searing it. Unless you toss it on a grill or hot plate it will not have any browning on the outside.