You don't have to do it forever, you just need to do it until you're happy with your weight. I've personally used this method of weight loss because I find it very difficult to exercise effectively when I'm running on a caloric deficit. I consumed 1,000 - 1,500 calories a day for ~2 months and lost ~20 pounds, then began living life normally again. I found it more sustainable than grinding out weight loss over a much longer period of time by exercising.