I’ve never heard that it was useful for fat loss. Any food is useful for fat loss in the sense that if you eat less, you can create the deficit to lose fat. In other words, I don’t believe whey helps you lose fat at all. Rather, it helps you supplement your protein intake so that you obtain more amino acids to recover from intense training.

However, if you are on a low carb diet extra protein can be good to ensure your muscles aren’t being broken down for energy. Since ketosis’ aim is to burn body fat for energy, you have to eat adequate protein during this process. Then a protein supplement could be useful.