I can't think of how you would "need" one, but it does make for some fun cooking. Here is one example:
Friends coming to our house over the holiday, they tell us they would like one of our "special" dinners. Well, wife and I both had to work that day ....... however, rib roast into the Sous Vide in the morning at 130F, get home, cook some sides for about an hour, sear the roast on a screaming hot gasser and voila!, dinner is served. The roast at a perfect medium rare.
Steaks ......
Also great for fish, chicken, whatever, you can't mess up because it cannot overcook. Another thing we love, pull out a vacuum sealed pack of pulled pork, ribs, brisket, whatever, set it at temp before work, come home, dinner is waiting. No more boiling pots, overcooking, etc. Yep, we use ours often. Then there's the cheap meat, long cook time advantage, more on that later.